[ bugfix ] Ty Nf a b -> Ty Ne a b

* It does not make sense for Ty to lift Normal forms: what are we
  expecting? lambda's & stuff? A type in normal form can either be
  a type constructor or a neutral term.
  This rot came from the first iterations of this formalisation
  where Ty and Nf used to be the same thing. There is no reason
  to retain it.