Implement IntoIterator for Events

[dtolnay]

The standard library typically implements IntoIterator<Item = T> for Collection<T> and IntoIterator<Item = &T> for &Collection<T>. In mio, Events has the second but not the first.

The owned iterator can be more convenient in some situations, like putting into a struct.

struct Bad {
    events: mio::Events,
    iter: mio::event::Iter<'??>,
}

struct Good {
    iter: mio::event::IntoIter,
}