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/runtime/haskell/PGF/Optimize.hs

@@ -20,7 +20,6 @@ import qualified Data.IntMap as IntMap
 import qualified GF.Data.TrieMap as TrieMap
 import qualified Data.List as List
 import Control.Monad.ST
-import GF.Data.Utilities(sortNub)
 
 optimizePGF :: PGF -> PGF
 optimizePGF pgf = pgf{concretes=fmap (updateConcrete (abstract pgf) .