Create a copair typeclass

[Jacoby6000]

I was speaking with @tpolecat in the gitter channel, and I was looking to generalize some things, where I'd like to accept Either, Xor and Validated. Rob mentioned that this would be called a Copair. I've done a very small (tiny) amount of work on this so far, but I know there's more. I'm working to fully implement this now. To provide an idea, my first pass implementation looked like this

  trait Copair[F[_,_]] {
    def left[A,B](a: A): F[A,B]
    def right[A,B](b: B): F[A,B]

    def fold[A,B,C](f: F[A,B])(fa: A => C, fb: B => C): C

    final def swap[A,B](f: F[A,B]): F[B,A] = fold(f)(right[B,A](_), left[B,A](_))
  }

  implicit class CopairOps[F[_,_]: Copair, A, B](pair: F[A,B]) {
    val ev = implicitly[Copair[F]]

    def fold[C](fa: A => C, fb: B => C): C = ev.fold(pair)(fa, fb)
    def swap: F[B,A] = ev.swap(pair)
  }

  implicit class CopairBuilderOps[A](a: A) {
    def left[F[_,_]: Copair, A, B](a: A): F[A,B] = implicitly[Copair[F]].left(a)
    def right[F[_,_]: Copair, A, B](b: B): F[A,B] = implicitly[Copair[F]].right(b)
  }

Based on the conversations I had with @tpolecat it sounds like you should be able to derive a Bifunctor and Bifoldable given a Copair. He also mentioned that there's probably a reason this thing doesn't exist, so this might be a dead end.

[etorreborre]

I am wondering if Copair is not actually a Coproduct as described in Alissa Pajer's presentation: https://vimeo.com/channels/flatmap2016/165852490.

[Jacoby6000]

Closing this issue in light of discussion on #976. Probably should've closed this ages ago 😅