Take impl AsFd instead of BorrowedFd

[A6GibKm]

This is more general, in particular the following will still compile.

let fd = something_implementing_as_fd;
let fd: &std::os::fd::BorrowedFd<'_> = &fd.as_fd();
ashpd::any_method_using_a_fd(fd).await?;

[A6GibKm]

Something annoying is that with this you cannot just use HashMap::new when one of generics is impl AsFd, see the docs change.

[bilelmoussaoui]

Needs a rebase

[A6GibKm]

Done.