[Runtime] Design away EncodableBodyContent (#30)

[Runtime] Design away EncodableBodyContent

### Motivation

Until #12 (Choose the serialization method based on content type), we were still looking for the right way to spell the various encoding/decoding methods that take into account both the type of the underlying schema, and the desired content type. In #12, we found something decent, but we could have simplified the spelling even further, getting rid of a whole closure invocation and a specialized type.

### Modifications

In this PR (in preparation for multiple content type support), we do the simplification by completely removing `EncodableBodyContent` and the associated closure trampoline that we used to propoagate the content type string. We now pass the string directly, and simplified all the helper functions that have to do with encoding body content.

Now, this is done in a backwards compatible way, so we really just deprecated all the existing methods, and the `EncodableBodyContent` type, but it'll all be cleaned up when we prepare for tagging the next breaking version.

### Result

The helper functions have a simpler spelling, which simplifies the generator and reduces the lines of code we have to generate.

(The associated generator PR will be coming in shortly.)

### Test Plan

I preserved all the tests for the deprecated variants, and added new unit tests for the new variants. The deprecated tests will also be cleaned up in the next breaking version.


Reviewed by: simonjbeaumont

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (api breakage) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#30

Author: czechboy0

Sources/OpenAPIRuntime/Base/EncodableBodyContent.swift

@@ -90,113 +90,113 @@ extension Converter {
     }
 
     //    | client | set | request body | text | string-convertible | optional | setOptionalRequestBodyAsText |
-    public func setOptionalRequestBodyAsText<T: _StringConvertible, C>(
-        _ value: C?,
+    public func setOptionalRequestBodyAsText<T: _StringConvertible>(
+        _ value: T?,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>
+        contentType: String
     ) throws -> Data? {
         try setOptionalRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertStringConvertibleToTextData
         )
     }
 
     //    | client | set | request body | text | string-convertible | required | setRequiredRequestBodyAsText |
-    public func setRequiredRequestBodyAsText<T: _StringConvertible, C>(
-        _ value: C,
+    public func setRequiredRequestBodyAsText<T: _StringConvertible>(
+        _ value: T,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>
+        contentType: String
     ) throws -> Data {
         try setRequiredRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertStringConvertibleToTextData
         )
     }
 
     //    | client | set | request body | text | date | optional | setOptionalRequestBodyAsText |
-    public func setOptionalRequestBodyAsText<C>(
-        _ value: C?,
+    public func setOptionalRequestBodyAsText(
+        _ value: Date?,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<Date>
+        contentType: String
     ) throws -> Data? {
         try setOptionalRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertDateToTextData
         )
     }
 
     //    | client | set | request body | text | date | required | setRequiredRequestBodyAsText |
-    public func setRequiredRequestBodyAsText<C>(
-        _ value: C,
+    public func setRequiredRequestBodyAsText(
+        _ value: Date,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<Date>
+        contentType: String
     ) throws -> Data {
         try setRequiredRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertDateToTextData
         )
     }
 
     //    | client | set | request body | JSON | codable | optional | setOptionalRequestBodyAsJSON |
-    public func setOptionalRequestBodyAsJSON<T: Encodable, C>(
-        _ value: C?,
+    public func setOptionalRequestBodyAsJSON<T: Encodable>(
+        _ value: T?,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>
+        contentType: String
     ) throws -> Data? {
         try setOptionalRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertBodyCodableToJSON
         )
     }
 
     //    | client | set | request body | JSON | codable | required | setRequiredRequestBodyAsJSON |
-    public func setRequiredRequestBodyAsJSON<T: Encodable, C>(
-        _ value: C,
+    public func setRequiredRequestBodyAsJSON<T: Encodable>(
+        _ value: T,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>
+        contentType: String
     ) throws -> Data {
         try setRequiredRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertBodyCodableToJSON
         )
     }
 
     //    | client | set | request body | binary | data | optional | setOptionalRequestBodyAsBinary |
-    public func setOptionalRequestBodyAsBinary<C>(
-        _ value: C?,
+    public func setOptionalRequestBodyAsBinary(
+        _ value: Data?,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<Data>
+        contentType: String
     ) throws -> Data? {
         try setOptionalRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertDataToBinary
         )
     }
 
     //    | client | set | request body | binary | data | required | setRequiredRequestBodyAsBinary |
-    public func setRequiredRequestBodyAsBinary<C>(
-        _ value: C,
+    public func setRequiredRequestBodyAsBinary(
+        _ value: Data,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<Data>
+        contentType: String
     ) throws -> Data {
         try setRequiredRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertDataToBinary
         )
     }

Sources/OpenAPIRuntime/Conversion/Converter+Client.swift

@@ -296,57 +296,57 @@ public extension Converter {
     }
 
     //    | server | set | response body | text | string-convertible | required | setResponseBodyAsText |
-    func setResponseBodyAsText<T: _StringConvertible, C>(
-        _ value: C,
+    func setResponseBodyAsText<T: _StringConvertible>(
+        _ value: T,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>
+        contentType: String
     ) throws -> Data {
         try setResponseBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertStringConvertibleToTextData
         )
     }
 
     //    | server | set | response body | text | date | required | setResponseBodyAsText |
-    func setResponseBodyAsText<C>(
-        _ value: C,
+    func setResponseBodyAsText(
+        _ value: Date,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<Date>
+        contentType: String
     ) throws -> Data {
         try setResponseBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertDateToTextData
         )
     }
 
     //    | server | set | response body | JSON | codable | required | setResponseBodyAsJSON |
-    func setResponseBodyAsJSON<T: Encodable, C>(
-        _ value: C,
+    func setResponseBodyAsJSON<T: Encodable>(
+        _ value: T,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>
+        contentType: String
     ) throws -> Data {
         try setResponseBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertBodyCodableToJSON
         )
     }
 
     //    | server | set | response body | binary | data | required | setResponseBodyAsBinary |
-    func setResponseBodyAsBinary<C>(
-        _ value: C,
+    func setResponseBodyAsBinary(
+        _ value: Data,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<Data>
+        contentType: String
     ) throws -> Data {
         try setResponseBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convertDataToBinary
         )
     }

Sources/OpenAPIRuntime/Conversion/Converter+Server.swift

@@ -347,22 +347,20 @@ extension Converter {
         return values
     }
 
-    func setRequiredRequestBody<T, C>(
-        _ value: C,
+    func setRequiredRequestBody<T>(
+        _ value: T,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>,
+        contentType: String,
         convert: (T) throws -> Data
     ) throws -> Data {
-        let body = transform(value)
-        headerFields.add(name: "content-type", value: body.contentType)
-        let convertibleValue = body.value
-        return try convert(convertibleValue)
+        headerFields.add(name: "content-type", value: contentType)
+        return try convert(value)
     }
 
-    func setOptionalRequestBody<T, C>(
-        _ value: C?,
+    func setOptionalRequestBody<T>(
+        _ value: T?,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>,
+        contentType: String,
         convert: (T) throws -> Data
     ) throws -> Data? {
         guard let value else {
@@ -371,7 +369,7 @@ extension Converter {
         return try setRequiredRequestBody(
             value,
             headerFields: &headerFields,
-            transforming: transform,
+            contentType: contentType,
             convert: convert
         )
     }
@@ -419,16 +417,14 @@ extension Converter {
         return transformedValue
     }
 
-    func setResponseBody<T, C>(
-        _ value: C,
+    func setResponseBody<T>(
+        _ value: T,
         headerFields: inout [HeaderField],
-        transforming transform: (C) -> EncodableBodyContent<T>,
+        contentType: String,
         convert: (T) throws -> Data
     ) throws -> Data {
-        let body = transform(value)
-        headerFields.add(name: "content-type", value: body.contentType)
-        let convertibleValue = body.value
-        return try convert(convertibleValue)
+        headerFields.add(name: "content-type", value: contentType)
+        return try convert(value)
     }
 
     func convertBinaryToData(