bundle2: move unpackheader closure into the class With the same argument as the other one, we move this closure into the `unbundlepart` class.

bundle2: move the fromheader closure into the class itself The class is now directly related to this header data. We can sanely move it on the class. I do not like closures very much...

bundle2: add an unbundle part responsible from unbundling part We have a new unbundle class and it is now responsible from extracting its own data. The top level bundler only extracts the header (to detect an end of stream marker) then leaves everything else to the `unbundlepart` class. The ultimate goal is to have `unbundlepart` responsible for lazily extracting its payload. This is mostly code movement.

bundle2: extract stream/unpack logic in an unpackermixin The coming `unbundlepart` will need the same kind of method than `unbundle20` for unpacking data from the stream. We extract them into a mixin class before the creation of `unbundlepart`.