Added split[N] to Coproduct

You are right: it looks like a lot don't do this. Perhaps it's not as important as I thought it was. I know that there have been times that something wouldn't compile until I changed a method's return type from X to X.Aux. It might have been in a different scenario, though.

It seems like declaring `Aux` as the return type is probably the right thing to do, and I agree with you that doing it in a refactoring PR is probably the right approach.