In the type checker Constraint and * are distinct; and the function
that takes the kind of a type should respect that distinction
(Trac #15971).

This patch implements the change:

* Introduce Type.tcTypeKind, and use it throughout the type
  inference engine

* Add new Note [Kinding rules for types] for the kinding
  rules, especially for foralls.

* Redefine
    isPredTy ty = tcIsConstraintKind (tcTypeKind ty)
  (it had a much more complicated definition before)

Some miscellaneous refactoring

* Get rid of TyCoRep.isTYPE, Kind.isTYPEApp,
  in favour of TyCoRep.kindRep, kindRep_maybe

* Rename Type.getRuntimeRepFromKind_maybe
  to getRuntimeRep_maybe

I did some spot-checks on compiler perf, and it really doesn't
budge (as expected).