Added an O(n log n) version of nub

The new nub is called nub', and it replaces the old sortNub which was 
not lazy and did not retain the order between the elements.

src/compiler/GF/Speech/RegExp.hs

@@ -53,7 +53,7 @@ isEpsilon (REConcat []) = True
 isEpsilon _ = False
 
 unionRE :: Ord a => [RE a] -> RE a
-unionRE = unionOrId . sortNub . concatMap toList 
+unionRE = unionOrId . nub' . concatMap toList 
   where 
     toList (REUnion xs) = xs
     toList x = [x]