There were several smallish bugs here:
 - We had too small an InScopeSet when rejigging GADT return types.
 - When adding the extra_tvs with a datatype kind signature, we
   were sometimes changing Uniques of an explicitly bound kind var.
 - Using coercionKind in the flattener got the wrong visibility
   for a binder. Now we just zonk to get what we need.

Test case: dependent/should_compile/RaeJobTalk