Template Haskell: allow type splices

At last!  Trac #1476 and #3177

This patch extends Template Haskell by allowing splices in
types.  For example

  f :: Int -> $(burble 3)

A type splice should work anywhere a type is expected.  This feature
has been long requested, and quite a while ago I'd re-engineered the
type checker to make it easier, but had never got around to finishing
the job.  With luck, this does it.

There's a ToDo in the HsSpliceTy case of RnTypes.rnHsType, where I
am not dealing properly with the used variables; but that's awaiting
the refactoring of the way we report unused names.