Add add_outlives method to Poller

[BrankoKrstic]

resolves #161

The previous implementation only implemented unsafe methods for registering file descriptors with Poller. This implements an add_outlives method that can guarantee safety by accepting a reference to an AsSource which outlives the Poller. Since file descriptors are guaranteed to be valid through the lifetime of the Poller, there is no need to ever delete to maintain soundness.

Please advise if I should change anything.

[notgull]

This is a breaking change, as Poller now has a lifetime parameter. I was envisioning something like this:

pub fn add_outlives<'poll, 'src, T: AsSource>(&'poll self, source: &'src T, interest: Event) -> io::Result<()>
where 'src: 'poll
 { /* ... */ }

This way the reference to the source is guaranteed to outlive the Poller.

[BrankoKrstic]

I tried this initially, but I don't think it will work if the source lifetime is not somehow tied to the Poller struct. The borrow checker will just shorten the poller lifetime inside add_outlives.

With the above example this compiles:

let poller = Poller::new().unwrap();

{
    let file = File::open("./file.txt").unwrap();
    let _ = poller.add_outlives(&file, Event::all(7));
    drop(file);
}
println!("{:?}", poller);

[notgull]

That's disappointing. I was under the impression that the borrow checker would catch this.

Still, thanks for checking!