Add support for AsFd

[VorpalBlade]

Rust 1.63 stabilised support for AsFd, which has the advantage over AsRawFd of tracking lifetimes for the file descriptor. Consider implementing support for this and possibly deprecating support for AsRawFd.

It doesn't look like the support for AsRawFd was ever in any release, so it should be fine to simply drop it without it being a semver issue.