Export something like splitTraverseSeq

I'm currently using it to implement `enumFromTo`-like functions for `Seq`, which demonstrate the same sort of performance advantage for incremental use as my proposed `zipWith` change. `splitTraverseSeq` would also be great for conversion from `Array` and `Vector` types. As it appears to be generally useful, I think we should export it in some form. We probably don't want to export a `Splittable` class, however, so we'll need to do something to work around that.