Add Functor#composeBifunctor

[armanbilge]

This PR proposes a different encoding of #4362 (note, which is not yet released). /cc @bpholt

Specifically, Cats has a precedent of offering various compose methods for creating nested instances, but I'm not sure if there is much precedent for offering these instances as implicits. Furthermore as noted in #4362 inference does not work very well for this implicit anyway.

So I think replacing the implicit with a composeBifunctor method may yield better and more consistent ergonomics i.e. Instead of:

Bifunctor[λ[(A, B) => F[(A, B)]]].leftMap(fab)(f)

now you would write:

Functor[F].composeBifunctor[(*, *)].leftMap(fab)(f)
// or
F.composeBifunctor[(*, *)].leftMap(fab)(f)

Thoughts?

[bpholt]

This seems pretty reasonable to me!