Add a `chroot` function?

Rust 1.56 stablizes [`std::os::unix::fs::chroot`](std::os::unix::fs::chroot). Should cap-std add a `chroot` function that can be called on a `Dir`?

It'd be implemented in terms of `fchroot` on platforms that have that, or `chroot` on `/proc/self/fd/{}` on Linux.

Start by reviewing the existing Dir API and platform-specific filesystem support. Compare Rust 1.56's std::os::unix::fs::chroot with fchroot and the Linux /proc/self/fd/{} approach described in the issue. Done means a consistent Dir-based chroot function with appropriate platform coverage and tests.

Written by the indexing model from the issue text.