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

[Henning Thielemann]

On Sat, 3 Feb 2018, Johannes Waldmann wrote:

> I propose this change for containers:
>
> have:  unions ::               [ Set a ] -> Set a
> want:  unions :: Foldable f => f (Set a) -> Set a

That's Foldable.fold.