Detect `List.foldl` used with `Set.union` in a suboptimal way

[miniBill]

List.foldl (\e a -> Set.union a e)
--> List.foldl (\e a -> Set.union e a)

The second form is going to be faster

[miniBill]

This also applies to Set.intersect, but in the other direction. We want the smallest Set as the first element, and that is going to be the accumulator, because it gets smaller as we fold, as opposed to the union case where it gets bigger