Support generics with consumption/inheritance in roles

[vrurg]

Make the following two constructs possible:

role R1[::T] does T {}
role R2[::T] is T {}

These were assumed possible, according to a comment in ParametricRoleHOW
code. But a few final touches needed to make them possible:

• traits must accept generics when applied to a parametric type
• typechecks must be done against instantiated typeobjects too,
  including parents
• additional validation for archetype of a consumed typeobject after
  instantiation since it bypassed trait validation

Spectests are passing.

[vrurg]

I currently see this rather useful for cases like:

role R[::T Foo] is T { }
class C does R[FooChild] { }

which would certainly be better than something like:

role R[::T Foo] { }
class C does R[FooChild] is FooChild { }

Especially when R declared in a module.