remove the `'static` bound on `Unpark`
remove the `'static` bound on `Unpark`

Because `Arc<Unpark>` defaults to `Arc<Unpark+'static>`, this bound was
useless. Moreover it hinders Rayon integration, since it means
we can't build a `Arc<Unpark+'foo>` that we transmute to a `Arc<Unpark>`
(since we manage the lifetimes).

Idolf/futures-rs,alexcrichton/futures-rs