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

[Johannes Waldmann]

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.