We were re-using the super-magical "state token" type (which has
VoidRep and is zero bits wide) for situations in which we simply want
to lambda-abstract over a zero-bit argument. For example, join points:

   case (case x of { True -> e1; False -> e2 }) of
      Red  -> f1
      Blue -> True

==>

  let $j1 = \voidArg::Void# -> f1
  in
  case x of
    True -> case e1 of
              Red -> $j1 void
              Blue -> True
    False -> case e2 of
              Red -> $j1 void
              Blue -> True

This patch introduces

   * The new primitive type GHC.Prim.Void#, with PrimRep = VoidRep

   * A new global Id GHC.Prim.voidPrimId :: Void#.
     This has no binding because the code generator drops it,
     but is used as an argument (eg in the call of $j1)

   * A new local Id, MkId.voidArgId, which can be lambda-bound
     when you need to lambda-abstract over it.

and uses them throughout.

Now the State# thing is used only when we need state!