Freezing the bundle2 format

[Pierre-Yves David]

I'm aiming at freezing the bundle2 binary format shortly.

If someone out there have been doing testing and spotted an issue with 
the current binary format, speak now.

The only possible change currently debated is related to the way we 
declare mandatory vs advisory part type.

My current plan is to send a patch freezing the format early next week.

-- 
Pierre-Yves David