Well. I should say zero length allocated object. This is mainly an api for
devs like @TooTallNate who are likely to find it useful to attach external
memory to any given object, and as a launch pad for my next patch.

There I added a dispose method to Buffers as well (which I'll show has some
awesome uses), but couldn't be done without removing the SlabAllocator.
Though all that definitely doesn't belong in this PR.