Implement AsRawFd for PpsDevice

[Erk-]

I want this to allow for more direct usage, more specifically I wanted to allow this usage:

use pps_time::PpsDevice;
use tokio::io::{Interest, unix::AsyncFd};

struct AsyncPps {
    inner: AsyncFd<PpsDevice>,
}

impl AsyncPps {
    fn new(pps: PpsDevice) -> std::io::Result<Self> {
        Ok(AsyncPps { inner: AsyncFd::new(pps)?, })
    }

    pub async fn wait(&self) -> std::io::Result<()> {
        let mut guard = self.inner.ready(Interest::READABLE).await?;
        guard.clear_ready();
        Ok(())
    }
}

I think this is nicer than to allow access to the underlying File.

[davidv1992]

This crate is primarily intended by us for pps in ntpd-rs, which has a bunch of minor things which we want to clean up eventually. Is this motivated by wanting to clean up there, or by a usecase you have for this crate?

[Erk-]

This was mainly motivated by a use case for the library (misusing pps as a global 10hz sync signal). And I wanted to be able to use it from Tokio.

#6 was just a small annoyance I had when I experimented with the library.

[davidv1992]

This looks like good changes. Just can't promise we will ever make a release with these, as we are currently doing rather large refactors around ntpd-rs and I'm not sure we are going to keep this crate in its current form.