bundle2: introduce a `parthandler` decorator Simple syntax sugar to register an handler for a new part type.

bundle2: first version of a bundle processing We now have a function that interpret part content. This is a version early version of this function. It'll see major changes in scope and API in future development. As for previous I'm just focussing on getting minimal logic setup. Refining will happen with real world usage.

bundle2: rename unbundle2 test command to statbundle2 We will introduce object to actually process the bundle, we need to keep the simplistic unbundle around for the test.