This is the last major addition to the kind-polymorphism story,
by allowing (Trac #5938)

 type family F a   -- F :: forall k. k -> *
 data T a          -- T :: forall k. k -> *
 type instance F (T (a :: Maybe k)) = Char

The new thing is the explicit 'k' in the type signature on 'a',
which itself is inside a type pattern for F.

Main changes are:

* HsTypes.HsBSig now has a *pair* (kvs, tvs) of binders,
  the kind variables and the type variables

* extractHsTyRdrTyVars returns a pair (kvs, tvs)
  and the function itself has moved from RdrHsSyn to RnTypes

* Quite a bit of fiddling with
     TcHsType.tcHsPatSigType and tcPatSig
  which have become a bit simpler.  I'm still not satisfied
  though.  There's some consequential fiddling in TcRules too.

* Removed the unused HsUtils.collectSigTysFromPats

There's a consequential wibble to Haddock too