Support type unification between C types and non-C types

[simonlindholm]

            elif not isinstance(x.ptr_to, Type) and not isinstance(y.ptr_to, Type):
                # TODO: deep resolve_typedefs (needs a typemap)
                if not equal_types(x.ptr_to, y.ptr_to):
                    return False
            else:
                # TODO: unify Type and CType (needs a typemap)
                return False

We need some additional context for this; maybe store typemap in Type or CType? Or use some "thread local state"-like pattern, since we have only a single typemap that we should use anyway?