Not applied to module-qualified types yet.  This lets the following
actually work:

    type opt 'a = Some 'a | None
    type int_opt = opt int

Up until this commit any attempt to use `int_opt` would cause a
unification failure because we weren't capturing `int` as an actual
parameter and associated variable name.  Now for each parameter that is
a concrete type we synthesize a variable name for it.

Refering to module.type_name with concrete type parameters now works.

The exact same block to link concrete types to synthetic type variables
occurred twice, now it's a single function.

Unifying two different ADTs when one is a member of the other called
unify/4 with the incorrect order of arguments.