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.