Struct support, performance improvements

• Immutable structs can now be used, you are no longer restricted to classes
• We no longer throw NSError-based errors, they're all enum ObjectBoxError now
• Strings that were created as NSStrings previously are now created as Swift Strings, which should reduce the number of unnecessary UTF-16/UTF-8 roundtrip conversions when Swift 5.1 arrives
• New projects are now set up with separate generated source files for each target by setup.rb.
• Binaries: Swift 5 ABI only