[Experimental] Implement JSON coding without using Foundation or `Codable`.

[grynspan]

This PR replaces most of our uses of JSONEncoder with a home-grown implementation. Types conform to the internal JSON.Serializable protocol and emit strings, numbers, dictionaries, arrays, etc. Unlike with Codable, there is no synthesized implementation here.

Pros:

• Less reliance on Foundation (ideally we drop the dependency entirely at some point);
• No need to use existentials to encode a value (allowing us to eventually support Embedded Swift); and
• The potential for more inlining as all the encoding work is done in-module.

Cons:

• Don't Repeat Yourself;
• This implementation doesn't come with a decoder (which is harder to write);
• The implementation of JSON.Serializable cannot be supplied by the compiler;
• More code means more technical debt and a higher maintenance burden;
• The implementation is less configurable/customizable than Foundation's; and
• The implementation is not a full Encoder conformance, so it cannot be used to reimplement CustomTestArgumentEncodable.

Resolves rdar://146964016.

Checklist:

• [ ] Code and documentation should follow the style of the Style Guide.
• [ ] If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

I'm going to redo this as an enum now that I have a better idea of the constraints we need to solve for.

[grynspan]

@swift-ci test