Allow reification of existentials and GADTs

Allow reification of existentials and GADTs

It turns out that TH.Syntax is rich enough to express even GADTs,
provided we express them in equality-predicate form.  So for
example

  data T a where
     MkT1 :: a -> T [a]
     MkT2 :: T Int

will appear in TH syntax like this

  data T a = forall b. (a ~ [b]) => MkT1 b
           | (a ~ Int) => MkT2

While I was at it I also improved the reification of types,
so that we use TH.TupleT and TH.ListT when we can.