Specifying concrete types as type parameters instead of vars fails typing

[j14159]

This fails, the typer tries to unify t_int and undefined:

module n
type opt 'a = Some 'a | None
type u = U opt int
let f () = U Some 1

This also fails with the same basic unification error:

module m
export_type t
type t 'a = T 'a

module n
type u 'a = U m.t 'a
let f () = U m.T 1

I haven't dug in too deeply yet but I expect what's happening is that when we do:

type option 'a = Some 'a | None
type something_else = option int

The parser has no idea that int is supposed to be assigned to a variable 'a (or any variable, for that matter) and so trying to get that variable from the vars proplist in an #adt{} yields undefined.

I think the fix might be pretty simple: when we look through the parameters given for a type that's a member of another type, we just manufacture a new type variable for each type expression that isn't already a type var.