Deprecation of the cbook module


Most of the methods in ``cbook`` are only meant for internal use. This PR deprecates the ``cbook`` module in favor of a new one which will not be mentionned in the user documentation. Moreover many of the methods present in the current ``cbook`` module are not used in matplotlib anymore. Hence, deprecating the module will help removing unused methods in the future.

I've proceeded into two steps:
1. As suggested by @mdboom, I deprecated the module cbook, and move the methods to a private module _cbook
2. I then moved all the used methods to a new module utils, and updated the code, documentation, tests and examples to reflect that change.

Hence we now have two new submodules: ``_cbook``, which contains methods that aren't used in matplotlib, and ``utils`` containing methods used in matplotlib.