bundle2: directly feed part to readbundle Now that part payload can be read like a stream, we can directly use it to feed the unbundle10 process.

bundle2: lazy unbundle of part payload The `unbundle` part gains a `read` method to retrieve payload content. This method behaves as a python file-like read method. The bundle-processing code is updated to make sure a part is fully consumed before another one is extracted. Test output changes because the debug output is even more interleaved now.

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.