rust-inner-revlog: always inline `get_entry` This is a very hot function.

rust-inner-revlog: derive Debug for IndexHeaderFlags

rust-inner-revlog: drop some outdated comment

rust-config: add more ways of reading the config These will be needed for future patches of this series to interpret more complex/different config values.

util: make buffer readonly There is no use of writable buffers anywhere in the code, and this helps us make sure we don't get into unsound territory when sharing memory with Rust. This `toreadonly` method was not available in Python 3.6, but we dropped the support for it earlier that week, so no need for any compatibility code.

setup: avoid the deprecated `distutils.spawn.find_executable` I noticed this was flagged with `DeprecationWarning` in py3.12 with `setuptools` 74.1.2, and it suggested `shutil.which()` instead. The signatures aren't the same, but the additional `mode` argument in the middle of the latter defaults to `os.F_OK | os.X_OK`, which maintains the same semantics.

setup: drop the hack to disable linker warning 4197 on Windows I don't see this when building on Windows with py3.8 or py3.12, so either the code was fixed, or (more likely) the compiler stopped warning about it some time after VS 2008. If we do have to put this back, it would probably be better to put a `#pragma` in a header file somewhere, and avoid `setuptools` technical debt.