wit/bindgen,cm: marshal & unmarshal for record types

[lxfontes]

Adds json struct tags to Records, based on wit names.

Also adds MarshalJSON & UnmarshalJSON to:

• cm.List
• cm.Tuple ( all of them )
• cm.Option
• cm.Variant

Example, encoding a complex record type:

  record response {
      headers: list<tuple<string, list<string>>>,
      http-code: u16,
      body: response-body
  }
  variant response-body {
    ok(list<list-element>),
    err(function-error),
    platform-err(platform-error)
  }
  record list-element {
      optional-int: option<u64>,
      optional-bool: option<bool>,
  }
  record function-error {
      error: string
  }
  record platform-error {
    code: string,
    message: string
  }

and filling it up:

	hdrVals := cm.ToList([]string{"value1", "value2"})
	hdrTuple := cm.Tuple[string, cm.List[string]]{
		F0: "header-name",
		F1: hdrVals,
	}
	headers := cm.ToList([]cm.Tuple[string, cm.List[string]]{hdrTuple})
	v := somefunctioninterface.Response{
		Headers:  headers,
		HTTPCode: 200,
		Body:     somefunctioninterface.ResponseBodyErr(somefunctioninterface.FunctionError{Error: "failed"}),
	}

Without this Pull Request:

{"Headers":{},"Body":{"RequiredParam":"required","OptionalParam":{}}}

With this Pull Request:

{"headers":[["header-name",["value1","value2"]]],"http-code":200,"body":{"err":{"error":"failed"}}}

Tuple handling

Tuples are encoded as json arrays with explicit nulls.

Tuple[string,int] -> [ "some string", 42 ]
Tuple3[string, *string, int] -> [ "some string", null, 42 ]

Variant handling

Variants are encoded as json dictionaries, so they can carry the variant data.

 record somerecord {
   myvariant: somevar,
 }
  variant somevar {
    ok,
    err(string),
  }

// JSON
{ "myvariant": { "ok": true }}
{ "myvariant": { "error": "error value" }}

Option handling

Options rely on explicit null for the "None" case.

{ "myoptional": null } // cm.None
{ "myoptional": "this" } // cm.Option[string]

If this direction seems reasonable I will add:

• Marshal/Unmarshal to cm.Result
• Encoding/Decoding Tests

[ydnar]

Amazing. This looks great so far. Thanks!

[ydnar]

Don't forget the space before the backtick, or just have a single string with HostLayout and the tag.

[ydnar]

Make sure to capture the name of the import. It might not be json.

[ydnar]

I think there might be a better way to do this.

Possibly by putting MarshalJSON and UnmarshalJSON in package cm, and generate only the minimal mapping from case name to case type.

[ydnar]

These methods should be on the inner list[T] struct.

[ydnar]

These methods should be on the inner struct.

[ydnar]

This should also check if buf == null.

[lxfontes]

👍 the null story here is interesting. will push it up as part of the breaking this PR into many

[lxfontes]

going to close this PR and move over to #239