HashSet: Add Hashable instance

Given that [a] has a Hashable instance and we already depend upon
hashable, it seems reasonable that HashSet should have one as well.

Author: bgamari

Data/HashSet.hs

@@ -65,7 +65,7 @@ module Data.HashSet
 import Control.DeepSeq (NFData(..))
 import Data.Data hiding (Typeable)
 import Data.HashMap.Base (HashMap, foldrWithKey)
-import Data.Hashable (Hashable)
+import Data.Hashable (Hashable(hashWithSalt))
 #if __GLASGOW_HASKELL__ < 709
 import Data.Monoid (Monoid(..))
 #endif
@@ -127,6 +127,9 @@ instance (Data a, Eq a, Hashable a) => Data (HashSet a) where
     dataTypeOf _   = hashSetDataType
     dataCast1 f    = gcast1 f
 
+instance (Hashable a) => Hashable (HashSet a) where
+    hashWithSalt = foldl' hashWithSalt
+
 fromListConstr :: Constr
 fromListConstr = mkConstr hashSetDataType "fromList" [] Prefix