Refactor: store [[PrimRep]] rather than [Type] in STG

StgConApp stored a list of types. This list was used exclusively
during unarisation of unboxed sums (mkUbxSum).
However, this is at a wrong level of abstraction:
STG shouldn't be concerned with Haskell types, only PrimReps.
Update the code to store a [[PrimRep]]. Also, there's no point in storing
this list when we're not dealing with an unboxed sum.