nodemap: have some python code writing a nodemap in persistent binary form
This python code aims to be as "simple" as possible. It is a reference
implementation of the data we are going to write on disk (and possibly,
later a way for pure python install to make sure the on disk data are up to
It is not optimized for performance and rebuild the full data structure from
the index every time.
This is a stepping stone toward a persistent nodemap on disk.
Differential Revision: https://phab.mercurial-scm.org/D7834