generalize type of Data.Set.unions from List to Foldable

Dear Libraries, I propose this change for containers: have: unions :: [ Set a ] -> Set a want: unions :: Foldable f => f (Set a) -> Set a and similar for unions in IntSet, Map, IntMap. Since this affects the public API, contributor guidelines require to go through this mailing list. See https://github.com/haskell/containers/issues/520 NB: the only other places in Data.Set (etc.) where a list occurs in an argument position are the from*List[With] functions. - J.W.