Add bifold to Bifoldable

[LukaJCB]

Something like this:

def bifold[F[_, _]: Bifoldable, A: Monoid, B: Monoid](fab: F[A, B]): (A, B) =
  bifoldMap(fab)(a => (a, Monoid[B].empty))(b => (Monoid[A].empty, b))

[Twizty]

Hi, is it still wanted? I'd like to implement that. I guess the previous attempt is abandoned.

[travisbrown]

This is closed in #3088.