Some methods are left teased out so it's easier to build a migration
extension from v1 packs to v2. I also anticipate those methods will be
of use if we get around to adding configurable compression engines
before we end up jettisoning packs.
I think it would make sense to register one-shot compress and
decompress methods on our compression engines in util.py, but
indygreg mentioned that might not be straightforward.