Stop implementing std's `MetadataExt`

[sunfishcode]

Rust is looking to seal std::os::unix::fs::MetadataExt:

rust-lang/rust#118916

As in #271 (which was before cap-std 1.0), cap-std can switch to defining its own copies of these traits. And since cap-std seems to be the only crate in the ecosystem that is affected, it should probably do this, to unblock std evolution.

However, this is a semver-incompatible change, so it'll require bumping cap-std to 3.0.

@cgwalters, who I know is particularly impacted by semver bumps

[cgwalters]

Since the cap-std 1.0 ➡️ 2.0 bump I did ostreedev/ostree#3027 fixes the most painful thing to bump from my PoV. So don't let me stand in the way of any Rust improvements!