jsx: allow locally abstract types and type constraints on @react.component

[glennsl]

This is a continuation of #487 that fixes the type annotations for locally abstract types. Because locally abstract types can't be defined in type annotations, they have to be replaced with plain type variables, and this has to be done recursively since they can occur anywhere in a type.

The only limitation I have intentionally imposed here is that extensions are not allowed in type annotations when locally abstract types are used, because that would require traversal of the entire AST, which is no fun.

As requested, here's a complete example:

module type Show = {
  type t
  let show: t => string
}

module Int = {
  type t = int
  let show = string_of_int
}

module Thing = {
  @react.component
  let make = (type a, ~model as module(Model: Show with type t = a), ~value: a) =>
      <div> {value->Model.show->React.string} </div>
}

let make = () =>
  <Thing model=module(Int) value=42 />

I hope this one covers all the bases :)

[cristianoc]

Looks good to me.
Leaving for @IwanKaramazow to take a final look.