Union typeclass for abstracting boxed union types

[leandrob13]

Would it make sense to work on a typeclass that can be defined for boxed union types like, but not limited to, Either or Ior? I have this idea in mind:

trait Union[U[_, _], A, B] extends ApplicativeError[U[A, ?], A] with Monad[U[A, ?]] {
  def isLeft: Boolean

  def isRight: Boolean

  def fold[C](Uab: U[A, B])(fl: A => C, fr: B => C): C

  def leftMap[C](Uab: U[A, B])(fl: A => C): U[C, B]
}

If it does make sense, I could work on the PR.

[peterneyens]

Also see #974 and #976.

[leandrob13]

@peterneyens I see the limitations, specially regarding Validated. Maybe define it only for Either and Ior but I guess it won't be enough to justify its addition to the library. I had a use for it but I guess I might have to rethink my use case.