`AsyncFd::new` and related constructors violate I/O safety

Summary

`tokio::io::unix::AsyncFd` provides safe constructors (`new`, `with_interest`, `try_new`, and `try_with_interest`) that accept any `T: AsRawFd` rather than requiring proof of ownership or validity of the file descriptor. Because `RawFd` implements `AsRawFd`, safe code can construct an `AsyncFd` from an arbitrary or closed file descriptor, or close/replace the underlying file descriptor while the `AsyncFd` is still alive.

This violates I/O safety in two ways:

1. `AsyncFd<T>` implements `AsFd` for all `T: AsRawFd` by calling `BorrowedFd::borrow_raw(self.as_raw_fd())`, allowing safe code to obtain an arbitrary `BorrowedFd<'_>` without owning the file descriptor. 2. Tokio's I/O driver stores a pointer to an internal allocation in the `epoll`/`kqueue` user data for registered file descriptors. If the underlying file descriptor is duplicated and then closed while an `AsyncFd` is alive, and the OS reuses that file descriptor number for a another IO resource on the same runtime, dropping the first `AsyncFd` deregisters the reused file descriptor number and frees the first while the original file description remains registered in `epoll`, leading to a use-after-free when a readiness event fires.

Starting in Tokio 1.51.6, 1.53.3, and 1.54.0, `AsyncFd::new`, `AsyncFd::with_interest`, `AsyncFd::try_new`, and `AsyncFd::try_with_interest` are deprecated in favor of the `unsafe` constructors `AsyncFd::register` and `AsyncFd::register_with_interest`. Callers must ensure that the file descriptor remains valid, open, and unchanged for the lifetime of the `AsyncFd`.