phases: move the binary decoding function in the phases module We move the decoding function near the encoding one in a place where they can be reused in other place (current target, 'exchange.py').

phases: move binary encoding into a reusable function We want to use binary phases for pushing and pulling. We extract the encoding function out of the bundle2 module first.

phases: use a Struct object for binary encoding and decoding We will move the binary encoding and decoding code to 'phases.py' in order to make it easier to reuse. First, let's cleanup it a bit.

discovery: avoid dropping remote heads hidden locally An extra post processing was added to recognize remote heads that are hidden locally as "common" instead of "unknown". However, this processing was removing such hidden heads from the remote heads sets. It had no impact because we used to pull phase information from all remote heads. This series will replace the phase pulling operation to a more efficient process but requires the unmodified pulled set information.