Support unboxed types within Safe Haskell

Currently, it is not possible to use unboxed types such as Int# or levity polymorphism (RuntimeRep, TYPE) with Safe Haskell, because GHC.Prim and GHC.Types are marked as unsafe.

As a first step, we could export RuntimeRep and TYPE from the module Data.Kind.

See also thread "Exposed # kinded variables + polykinded Prelude classes?" at the libraries mailing list, Oct 2019.

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