Fix Struct issues

[mitschabaude]

This fixes type inference in the following code example:

class A extends Struct({a: Field}) {
  print() {
    console.log(this.a.toString());
  }
}

class B extends Struct({a: A, b: Bool}) {
  print() {
    this.a.print(); // FIXED!
  }
}

The type inference is changed in a way that it tries to not affect cases where inferring a class type already worked (e.g., CircuitValues like UInt32)

EDIT: this PR now is about fixing more issues with Struct that came up

• closes When the custom Struct contains member methods, there will be a type mismatch when using Circuit.switch and Circuit.witness #576
• makes all (I hope) our APIs work with customized Structs
• fixes Struct.fromJSON

[Trivo25]

lgtm, just left a small comment

[Comdex]

@mitschabaude Hey, can you export these two types for external use? This allows developers to use these types to support various custom Structs as parameters of generic methods.

type FlexibleProvable<T> = Provable<T> | Struct<T>;
type FlexibleProvablePure<T> = ProvablePure<T> | StructPure<T>;