rust-files: separate the listing of files from a revset and a revision We won't need to parse a revset all the time, and an upcoming patch will make use of this new util.

rust-files: add a `Sync` bound to the matcher This enables us to use this code in multithreaded environements, which we very much want to do.

rust-files: also return filenode and flags when listing a revision's files This is going to be useful when implementing parts of `update` and makes it so we don't have to fetch the manifest and each entry twice.

rust-manifest: encode flags as `Option<NonZeroU8>` This makes the compiler use the niche optimization for all flags: since 0 is not a valid representation of any flags, we can use 0 as a replacement for `None`, which reduces memory footprint and could yield a little performance improvement over many iterations.