I/O safety.

[sunfishcode]

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: #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