traverse_ could be on UnorderedFoldable

[johnynek]

Also, we should probably implement traverse_ in terms of foldMap not foldRight

def traverse_[A, B](fa: F[A])(fn: A => G[B])(implicit G: Applicative[G]): G[Unit] =
   foldMap(fn.andThen(_.unit))(Applicative.monoid[G, Unit])

foldRight is not the most efficient thing in the world since it leverages Eval (although, it may be that implementation is stack safe and the above is not.

[johnynek]

Well, I guess it would have to take a CommutativeApplicative so maybe it is not the same function on UnorderedFoldable.

[LukaJCB]

Yeah, I think you're right in that regard, probably should also think about how #2516 plays into this.