Type-class support MVP

[yairchu]

This issue is about the ability to implement languages with type-classes. Such that LangB will have another term constructor BInstance which evaluates to a type-class instance according to its type (such as [Int] -> Json).

• The infer of "instance" nodes should add a type-class constraint
  • For evaluation/compilation: it might be useful for the InferOf to include a reference to the term's corresponding type-class constraint so that the type-class instance can be looked up from it
• The generated constraints can probably be resolved/simplified in a post-process after the inference, that matches them with existing instances. For example [Int] -> Json can be resolved by a -> Json => [a] -> Json and Int -> Json.
• TODO (flesh-out): We should consider the interactions with let-generalization and to-nom