Please consider supporting AsFd

[joshtriplett]

Currently, most methods async_io::Async requires the type it wraps to implement AsRawFd, or AsRawSocket on Windows.

The standard library now supports the safe alternative AsFd (or AsSocket on Windows), and async-io has some support for this. However, it doesn't seem possible to create an Async<T> for a type that implements AsFd but not AsRawFd.

I have types I'd love to use Async with, and I'd like to drop the implementation of AsRawFd for those types. (This would also eliminate unsafe code.)

Please consider adding a constructor for Async<T> where T: AsFd.

[notgull]

cc smol-rs/polling#38

I definitely want to move in this direction, especially since doing this would allow us to make this crate forbid(unsafe_code) and would make things safer to use overall. The best way of doing this is to wait for Debian Bookworm to release and then bump the MSRV of this crate to 1.63.

[notgull]

Closed by #142