hg-core: add a complete VFS This will be used from Python in a later change. More changes are needed in hg-core and rhg to properly clean up the APIs of the old VFS implementation but it can be done when the dust settles and we start adding more functionality to the pure Rust VFS.

hg-core: add fncache module For now it's only a super simple trait. It will be used for calling back into Python soon, and later will be fleshed out into a full fncache.

rust: populate mmap by default if available See 522b4d729e89edc76544fa549ed36de4aea0b7fb for more details. Background population to follow in a later patch.

rust-changelog: switch away from deprecated APIs for datetime use This was caught by clippy, nothing was changed aside from some light API changes.

revlog: add the glue to use the Rust `InnerRevlog` from Python The performance of this has been looked at for quite some time, and some workflows are actually quite a bit faster than with the Python + C code. However, we are still (up to 20%) slower in some crucial places like cloning certain repos, log, cat, which makes this an incomplete rewrite. This is mostly due to the high amount of overhead in Python <-> Rust FFI, especially around the VFS code. A future patch series will rewrite the VFS code in pure Rust, which should hopefully get us up to par with current perfomance, if not better in all important cases. This is a "save state" of sorts, as this is a ton of code, and I don't want to pile up even more things in a single review. Continuing to try to match the current performance will take an extremely long time, if it's not impossible, without the aforementioned VFS work.

changelog: also set the general delta config flag in the data config This duplication is dubious, but it's a decision to be made at a later date, this is the fix.

rust-index: use `IndexEntry::offset` to compute read segments This only matters for inline revlogs where the impact is debatable, but this is what the C index does.

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.

rust-index: fix the computation of data start This was falling into place instead of being correct, we clean up the logic by differenciating the on-disk offset and the actual start of the data more cleanly.

rust-index: return an error on a bad index header This is more idiomatic and allows us to better handle the problem later.