This patch implements part 1 of GHC Proposal #281,
introducing explicit `type` patterns and `type` arguments.

Summary of the changes:

1. New extension flag:
     RequiredTypeArguments

2. New user-facing syntax:
     `type p` patterns    (represented by EmbTyPat)
     `type e` expressions (represented by HsEmbTy)

3. Functions with required type arguments (visible forall)
   can now be defined and applied:
      idv :: forall a -> a -> a    -- signature   (relevant change: checkVdqOK in GHC/Tc/Validity.hs)
      idv (type a) (x :: a) = x    -- definition  (relevant change: tcPats in GHC/Tc/Gen/Pat.hs)
      x = idv (type Int) 42        -- usage       (relevant change: tcInstFun in GHC/Tc/Gen/App.hs)

4. template-haskell support:
      TH.TypeE corresponds to HsEmbTy
      TH.TypeP corresponds to EmbTyPat

5. Test cases and a new User's Guide section

Changes *not* included here are the t2t (term-to-type) transformation
and term variable capture; those belong to part 2.