Add poly support

[ProgramingIsTheFuture]

For polymorphic types:

type 'a t = {
  my_field : 'a;
}
val default: 'a -> unit -> 'a t

I want to avoid adding a new parameter if the type inside is 'a option (the default is None).

I want to improve the following

type my_type = any

type eg =
  | Some of my_type

What the ppx_default does, in this case, is calling the default_my_type ().
Need to fix the case of that type being polymorphic.