Kind TyCons: require KindSignatures, not DataKinds

  Uses of a TyCon in a kind signature required users to enable
  DataKinds, which didn't make much sense, e.g. in

    type U = Type
    type MyMaybe (a :: U) = MyNothing | MyJust a

  Now the DataKinds error is restricted to data constructors;
  the use of kind-level type constructors is instead gated behind
  -XKindSignatures.

  This patch also adds a convenience pattern synonym for patching
  on both a TyCon or a TcTyCon stored in a TcTyThing, used in
  tcTyVar and tc_infer_id.

  fixes #20873