RFC/WIP: Introduce capnp-json for JSON code compatible with c++ default capnproto implementation

[puremourning]

As discussed in #68, I'm sharing this very rough cut of JSON codec support for input.

Some assumptions I made, which influenced the implementation, but could be changed depending on your view:

• avoid third party crates: json parser/encoder rolled from scratch, base64/hex encoders rolled from scratch
• should be its own crate in the workspace (per comment in Consider merging with Serde #68)
• std is ok for this crate
• perfect is the enemy of good (just kidding, or rather trying to soften the blow, as this implementation is not art)

What's supported:

• Compatible with the default encoding provided by the upstream c++ implementation I believe
• Encode and Decode to/from String
• Annotations provided by upstream: flatten, discriminator, name
• base64, hex and array of numbers encodings for Data fields.
• a smattering of tests. coverage is probably not yet sufficient.

The best doc on the actual encoding is the json.capnp which is taken directly from upstream. We don't use all of it, but I kept it unmodified anyway.

What's not supported, and what deviates from the upstream implementation:

• Any way to customise the encoding/decoding. The c++ implementation has a sort of codec plugin which allows users to supply arbitrary codecs for fields.. This is probably most useful for Data fields.
• Speed. This is not an optimised parser, serialiser or anything. It's the simplest way to just do something that works.
• Arbitrary "JsonValue" types in messages. This is due to a difference in the way the codecs work. The c++ implementation uses a JsonValue (capnp) as an intermediary format for any value type whether encoding or decoding. This relies on Orphans (which we don't support) and seems to me to be an unnecessary overhead. In this implementation we go direct from Reader -> JSON string. For decoding, we still need an intermediary format, so we use a JsonValue of our own (a Rust enum) and thus JSON string -> JsonValue enum -> Builder.
• There's a lack of comments and general polish; to be done if we agree on the bigger picture items.

[codecov]

Codecov Report

❌ Patch coverage is 50.87789% with 1231 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.98%. Comparing base (ab342b3) to head (8516b73).
⚠️ Report is 235 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp-json/src/json_capnp.rs	4.79%	1013 Missing ⚠️
capnp-json/src/decode.rs	68.76%	164 Missing ⚠️
capnp-json/src/data.rs	82.41%	16 Missing ⚠️
capnp-json/src/lib.rs	77.27%	15 Missing ⚠️
capnp-json/test/json.rs	97.56%	13 Missing ⚠️
capnp-json/src/encode.rs	95.57%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
- Coverage   51.64%   50.98%   -0.66%     
==========================================
  Files          69       76       +7     
  Lines       33735    34838    +1103     
==========================================
+ Hits        17422    17762     +340     
- Misses      16313    17076     +763     

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