rhg: add rhg crate
The goal of rhg is to speedup some of hg's commands when possible by bypassing
python entirely for the time being.
It is by no means a replacement for hg as it will not support extentions or
configuration and implement only a subset of hg's commands and options.
Only use rhg if you understand what the tradeoffs are.
Differential Revision: https://phab.mercurial-scm.org/D8610