Refactor snakeoil.compression ArComp, _Archive, Compressed into a clear ontology

These base classes are doing registry type functionality- which is good- but the way it's done and intermixing classes on the fly (and lack of clarity on what is/isn't mixin) is actually hurting my brain, and I live/die by meta.

The code works, but it needs to be simplified so it's clear exactly how this flows.
