Implement [GHC Proposal 203](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst), the Pointer Rep proposal. I'm going to start on this, but I'd like to solicit some input on one design decision. Internally, GHC uses both a `PrimRep` type and a `RuntimeRep` type. The relationship between the two is well documented in `Note [RuntimeRep and PrimRep]` and `Note [Getting from RuntimeRep to PrimRep]`. The change that should be made to `RuntimeRep` is obvious. It is specified in the proposal. The change to `PrimRep` is (to me) less obvious. Currently, `PrimRep` is: data PrimRep = LiftedRep | UnliftedRep | IntRep ... I suspect that it would be a mistake to change `PrimRep` to: data Levity = Lifted | Unlifted data PrimRep = BoxedRep Levity | IntRep ... I think it should be left alone. Does anyone more familiar with the runtime representation code have some intuition for the right change here?