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.

rust-repo: add a method to set the current parents This will be useful when we start writing an `hg update` fastpath.

rust-repo: add a method to get a `Node` from a `Revision` to the `Repo` This is going to be more and more useful as we start writing higher-level code

rust-utils: move the `filesystem_now` function to a util This is going to be useful for an upcoming `hg update` fastpath.