util: support None size in chunkbuffer.read() When no size is provided, read the whole buffer. This aligns with the usual behavior of `read()` in python.

bundle2: lazily iterate over bundle parts in the test We used to create a list to know the number of parts in the bundle. This prevents any lazy reading as planned for real usage. The list creation is dropped. Some test output changed as debug output is now interleaved with command output.

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`.