We now always check against an expected kind. When we really don't know what
kind to expect, we match against a new meta kind variable.

Also, we are more explicit about tuple sorts:
  HsUnboxedTuple                  -> Produced by the parser
  HsBoxedTuple                    -> Certainly a boxed tuple
  HsConstraintTuple               -> Certainly a constraint tuple
  HsBoxedOrConstraintTuple        -> Could be a boxed or a constraint
                                  tuple. Produced by the parser only,
                                  disappears after type checking