fncache: add attribute to check whether we're using dotencode This will make it easy to know if we can use the Rust implementation that doesn't support older forms of encoding.

fncachestore: add typing information This helps with autocomplete.

fncache: refactor load check into a property This makes the intent more obvious new callers less prone to error.

hg-core: add FnCacheVFS This will allow us to only call back into Python to add items to the fncache, which should save us a lot of FFI overhead. This is also of course a stepping stone for more pure Rust work.

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.