addLeft/addRight convenience methods for Iors?

[keirlawson]

Whilst contributing to a cats-based codebase that makes extensive use of Iors (typelevel/grackle#96) I found myself needing a "addLeft" function, which given an Ior would either combine the new value with the existing LHS or in the case where it was a Right, turn it in to a Both with the new value on the left, ie

  def addLeft[A: Semigroup, B](ior: Ior[A,B], a: A): Ior[A, B] = ior match {
    case Ior.Right(r) => Both(a, r)
    case Both(l, r) => Both(l combine a, r)
    case Ior.Left(l) => Ior.left(l combine a)
  }

Would this be something that would make sense to include within Cats itself?

[LukaJCB]

Sounds like a good addition to me :)