Strict.differenceWith: Relax Hashable constraint

Fixes an oversight in #542.

Author: sjakobi

Data/HashMap/Internal/Strict.hs

@@ -620,7 +620,7 @@ traverseWithKey f = go
 -- encountered, the combining function is applied to the values of these keys.
 -- If it returns 'Nothing', the element is discarded (proper set difference). If
 -- it returns (@'Just' y@), the element is updated with a new value @y@.
-differenceWith :: Hashable k => (v -> w -> Maybe v) -> HashMap k v -> HashMap k w -> HashMap k v
+differenceWith :: Eq k => (v -> w -> Maybe v) -> HashMap k v -> HashMap k w -> HashMap k v
 differenceWith f = HM.differenceWithKey $
   \_k vA vB -> case f vA vB of
      Nothing -> Nothing