Add splitMap function to Swift standard library

This is a thing that we want to be possible with variadics (and I would probably not add it to the standard library proper until it could be done in such a manner), so this points at variadics features or documentation that's missing. Can you either file a bug report or make a separate thread focused on this?

In the meantime, this might make sense for algorithms. In some sense it's "just" a reduce, but half of algorithms is "just a reduce", so I think I'm OK with that. @nnnnnnnn?

(There's also the question of using this as a backdoor introduction for Either that would have to be resolved somehow...)