bundle2: use new compression engine API for compression Now that we have a new API to define compression engines, let's put it to use! The new code stores a reference to the compression engine instead of a low-level compressor object. This will allow us to more easily transition to different APIs on the compression engine interface once we implement them. As part of this, we change the registration in bundletypes to use 'UN' instead of None. Previously, util.compressors had the no-op compressor registered under both the 'UN' and None keys. Since we're switching to a new API, I don't see the point in carrying this dual registration forward.