I am still not a fan of these random classes/interfaces sitting in `rx.util`, but I can't think of anything better to do with them.

I don't really want them in `rx` and they don't seem to warrant their own package, but they seem different than `util`.

We could put them in `Observable` as inner classes but that seems like a slipper slope of creating clutter.

Does anyone else have a better idea of what to do with them?