bundle2: update part creators to ``addparam`` when relevant In some cases, the use of ``addparam`` makes code much clearer.

bundle2: introduce a ``addparam`` method on part We make it easier to add new parameters after the part creation. As for the ``data`` attribute we make sure the part generation has not begun yet.

bundle2: the ability to set ``data`` attribute of the part is now official We make it safe to set the data attribute after part creation. It is an allowed operation as long as the part has not started to be generated.

bundle2: introduce a ReadOnlyPartError exception As we will introduce functions to alter already created parts, we need a proper exception to raise when code tries to alter a part that cannot be altered anymore.

bundle2: warn about error during initialization in ``newpart`` docstring As we are moving toward being able to alter a part after its creation, we need to make the implication of the part being already part of the bundle2 clear.

bundle2: track life cycle of parts We introduce a ``_generated`` attribute on parts. Coming changesets will make it easier to update a part's contents after its creation. We need a way to track if the part is still open to modification or if it is currently being generated and should not be touched anymore. As a bonus, we can now detect and crash if someone manages to write bogus code to get a part generated twice.

bundle2: update all ``addpart`` callers to ``newpart`` The new method is what we want in all current cases.

bundle2: have ``newpart`` automatically add the part to the bundle The created part is automatically added to the bundle as this is most certainly the intent of the user code.

bundle2: add a ``newpart`` method to ``bundle20`` Creating new parts is the most common operation people do when exposed to a bundler. We create a dedicated method on the bundler object for it. This will simplify the code and also avoid having to import the ``mercurial.bundle2`` module in multiple places. One part creators have been updated for testing purpose.

bundle2: small doc update on the bundler The `bundle20` class contains methods to help define the content and methods to generate the actual stream. We add small doc headers to help distinguish between the two.