Auto merge of #87329 - sunfishcode:sunfishcode/io-safety, r=joshtriplett

Auto merge of #87329 - sunfishcode:sunfishcode/io-safety, r=joshtriplett

I/O safety.

Introduce `OwnedFd` and `BorrowedFd`, and the `AsFd` trait, and
implementations of `AsFd`, `From<OwnedFd>` and `From<T> for OwnedFd`
for relevant types, along with Windows counterparts for handles and
sockets.

Tracking issue: <https://github.com/rust-lang/rust/issues/87074>

RFC: <https://github.com/rust-lang/rfcs/blob/master/text/3128-io-safety.md>

Highlights:
 - The doc comments at the top of library/std/src/os/unix/io/mod.rs and library/std/src/os/windows/io/mod.rs
 - The new types and traits in library/std/src/os/unix/io/fd.rs and library/std/src/os/windows/io/handle.rs
 - The removal of the `RawHandle` struct the Windows impl, which had the same name as the `RawHandle` type alias, and its functionality is now folded into `Handle`.

Managing five levels of wrapping (File wraps sys::fs::File wraps sys::fs::FileDesc wraps OwnedFd wraps RawFd, etc.) made for a fair amount of churn and verbose as/into/from sequences in some places. I've managed to simplify some of them, but I'm open to ideas here.

r? `@joshtriplett`