Proposal: export more from Data.Kind

[Krzysztof Gogolewski]

Hello,

I'd like to export TYPE, RuntimeRep(..), Multiplicity(..) from Data.Kind.
(Multiplicity is the Linear Haskell type - One and Many.)

Currently you have to import GHC.Exts / GHC.Types which conflicts with
Safe Haskell. I think both levity and linearity polymorphism
should be available under Safe Haskell.

Data.Kind already contains Constraint and Type.

-Krzysztof