Go: Use private interfaces instead of generating structs for discriminating types

[diamondburned]

Proposal

This issue proposes that for the following JSON type definition:

{
  "Thing": {
    "discriminator": "type",
    "mapping": {
      "a": {
        "properties": {}
      },
      "b": {
        "properties": {}
      },
      "c": {
        "properties": {}
      }
    }
  }
}

jtd-codegen should generate the following Go code:

type Thing struct {
	Type string `json:"type"`

	// Value can be the following types:
	//
	//    - [A]
	//    - [B]
	//    - [C]
	//
	Value thingValue `json:"-"`
}

func (t *Thing) UnmarshalJSON(data []byte) error
func (t *Thing) MarshalJSON() ([]byte, error)

type thingValue interface {
	isThing()
}

type ThingA struct{}
type ThingB struct{}
type ThingC struct{}

func (ThingA) isThing() {}
func (ThingB) isThing() {}
func (ThingC) isThing() {}

The user would consume the API like so:

var t Thing

switch v := t.Value.(type) {
case ThingA:
	log.Println("thing contains type A")
case ThingB:
	log.Println("thing contains type B")
case ThingC:
	log.Println("thing contains type C")
default:
	log.Println("thing contains unknown type")
}

If the user makes a mistake in the type-switch, the compiler will complain:

var t Thing

switch v := t.Value.(type) {
case ThingA:
	log.Println("thing contains type A")
case string:
	// does not even compile
}

This issue is an alternative to issue #49.

Rationale

Pros:

• Using interfaces instead of flat structs can sometimes takes up less space in
  memory if the mapping is large.
• The user can use type-switches to determine the type of the value, which is
  more idiomatic and safer in Go than using a string field.

Cons:

• The magic field's name Value is not ideal. This is done because we can't
  have custom marshalers and unmarshalers on an interface.
• The magic field has an unexported type. Showing an unexported type in an
  exported type is not ideal.
• It's not explicit which types actually satisfy TValue outside a single
  comment. This might be confusing to users.

[diamondburned]

If I can get this project to build locally in Nix, I will probably file a PR for this.