Add FutureExt::wait

[yoshuawuyts]

Ref: smol-rs/async-task#16

Design Sketch

trait FutureExt: Future {
    /// Wait for the `Future` to complete execution.
    pub fn wait(self) -> <Self as Future>::Output;
}

[yoshuawuyts]

Rationale here: smol-rs/async-task#16 (comment), but after having researched it further I no longer think this is a good idea. Instead if we want to optimize specific sync variants of futures we should be able to provide specializations for those futures in thread::block_on.