Increase extensibility of pickle5's C-Pickler

[pierreglaser]

Follow up on: cloudpipe/cloudpickle#253 (comment)
It would be nice to have cloudpickle use the pickle5-backport C-Pickler on Python < 3.8, for 2 reasons:

• use protocol 5 on older Python versions.
• make cloudpickle faster by using pickle5's CPickler instead of the pythonic Pickler on those older versions.

@pitrou would you be ok with integrating the recent changes of the Python3.8's pickle into pickle5?

[pitrou]

@pierreglaser @suquark @pcmoritz I've just released version 0.0.9 on PyPI, FYI.

[pierreglaser]

Thank you!