rust-revlog: add a Rust-only `InnerRevlog`
This mirrors the Python `InnerRevlog` and will be used in a future patch
to replace said Python implementation. This allows us to start doing more
things in pure Rust, in particular reading and writing operations.
A lot of changes have to be introduced all at once, it wouldn't be very
useful to separate this patch IMO since all of them are either interlocked
or only useful with the rest.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).