[MERGE][RFC] Add simple revision serializer based on RIO.

[Stephen J. Turnbull]

Alexander Belchenko writes:

 > Revision objects are stored in pack files that again uses very
 > complex binary format, not human-readable. Perhaps I'm missing
 > something important?

Maybe; it's certainly true that git and Mercurial do the same thing.
Both of them do it for efficiency.  Packs are more efficient for
transmission (enough so that the git not-totally-stupid-server builds
them on the fly for cloning), and on local disk as well (way fewer
stats).