Improve SpecConstr

This commit fixes SpecConstr so that it can see the effect of
enclosing case expressions properly.  That's what the "cons" field
in ScEnv is for.

As a result, consider this function:

  data AccessPath = Cont  AccessPath
		  | Value Int

  demandAll n ap@(Cont (Value (I# i1)))
    = case n of
	0     -> i1
	other -> i1 +# demandAll (n-1) ap

SpecConstr now successfully compiles it to this:

  $s$wdemandAll
    = \ i1 :: PrelGHC.Int# sc :: PrelGHC.Int# ->
      case sc of ds {
	0 -> i1;
	__DEFAULT -> PrelGHC.+# i1 (Foo.$s$wdemandAll i1 (PrelGHC.-# ds 1))
      }

with the rule

 "SC:$wdemandAll1" __forall i1 :: PrelGHC.Int# ,
			    sc :: PrelGHC.Int# .
	Foo.$wdemandAll sc (Foo.$wCont (Foo.$wValue (PrelBase.$wI# i1)))
	= Foo.$s$wdemandAll i1 sc ;