Previously I had it so that dv_kvs and dv_tvs didn't
overlap.  Now they can, and quantifyZonkedTyVars
removes the former from the latter.  This is more
economical, and in fact there was a bug where the
invariant wasn't re-established.

It's much easier to allow dv_kvs and dv_kvs to overlap,
and to eliminate the overlap in TcMType.quantifyZonkedTyVars