Don't use TypeInType on recent GHCs

On GHC 8.6 or later, `TypeInType` is simply an alias for `DataKinds` +
`PolyKinds`.  What's more, `TypeInType` emits deprecation warnings with GHC 9.6
or later, so this patch guards the use of `TypeInType` behind CPP to avoid the
warnings.

src/Data/Type/RList.hs

@@ -1,6 +1,10 @@
 {-# LANGUAGE TypeOperators, EmptyCase, EmptyDataDecls, RankNTypes #-}
 {-# LANGUAGE TypeFamilies, DataKinds, PolyKinds, KindSignatures #-}
-{-# LANGUAGE GADTs, TypeInType, PatternGuards, ScopedTypeVariables #-}
+{-# LANGUAGE GADTs, CPP, PatternGuards, ScopedTypeVariables #-}
+
+#if __GLASGOW_HASKELL__ < 806
+{-# LANGUAGE TypeInType #-}
+#endif
 -- |
 -- Module      : Data.Type.RList
 -- Copyright   : (c) 2016 Edwin Westbrook