extend AsyncFdReadyGuard method rettypes lifetimes

[axelf4]

Motivation

The implicit elided lifetimes of the AsyncFd references in return types of methods on AsyncFdReadyGuard resolved to that of &self, which is needlessly small.

Solution

Change the lifetimes of the references. Should not be a breaking change thanks to subtyping.

---

On a parallel note, I would also like to get an opinion on whether this pull request should be made to include a new into_try_io() method on AsyncFdReadyMutGuard:

pub fn into_try_io<R>(
    self,
    f: impl FnOnce(&'a mut Inner) -> io::Result<R>,
) -> Result<io::Result<R>, TryIoError> {
    let AsyncFdReadyMutGuard {
        async_fd,
        mut event,
    } = self;
    let result = f(async_fd.inner.as_mut().unwrap());

    match result {
        Err(err) if err.kind() == io::ErrorKind::WouldBlock => {
            if let Some(event) = event.take() {
                async_fd.registration.clear_readiness(event);
            }
            Err(TryIoError(()))
        }
        result => return Ok(result),
    }
}

since the currently existing try_io() method is limiting when writing streaming streams, as the result of the closure cannot reference anything that outlives the guard handle. One example where this perhaps would have been useful is in the implementation of the capture-stream feature of the pcap crate.

[Darksonn]

Note that we cannot make an analogous change for AsyncFdReadyGuardMut because of the uniqueness requirement of mutable references.