Improve pruning of case alternatives to account for GADTs

Improve pruning of case alternatives to account for GADTs

Consider

  data T a where
    T1 :: T Int
    T2 :: T Bool
    T3 :: T Char

  f :: T Bool -> Int
  f x = case x of
	  DEFAULT -> ...
	  T2 -> 3

Here the DEFAULT case covers multiple constructors (T1,T3), but none
of them can match a scrutinee of type (T Bool).  So we can prune away
the default case altogether.

In implementing this, I re-factored this bit of the simplifier, elminiating
prepareAlts from SimplUtils, and putting all the work into simplAlts in
Simplify

The proximate cause was a program written by Manuel using PArrays