Implement GHC Proposal 203, 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?