bundle20: extract core payload generation in its own function We are about to allow compressing the core of the bundle2. So we extract the generation of this bits in its own parts to make this compression phases easier in a later changesets.

unbundle20: allow registering handlers for stream level parameters As a comment in the code have been asking for, it is now possible to register piece of code that handle parameters for the stream. I've been wondering is such function should be class methods or not. I eventually went for externally decorated methods to stick with the culture of extensibility from an extensions that apply to bundle2.

bundle2: allow to specify unsupported value on error A client may support an argument but not some of its values (eg: coming "compression" parameters). We allow this case to be carried in the exception.