rust: block_on panics if the future yields before any waitable is registered

[dsamp]

This code:

fn main() {
    wit_bindgen::block_on(async {
        wit_bindgen::yield_async().await;
    });
    println!("done");
}

currently panics with the following message:

thread 'main' (1) panicked at .../wit-bindgen-0.62.0/src/rt/async_support.rs:671:38:
called Option::unwrap() on a None value

It isn't specific to yield_async. Any wake that happens while the task set is being polled, before the first host call, hits it too. Putting a host call first avoids it, e.g. awaiting wasip3::clocks::monotonic_clock::wait_for(1_000_000). After that, the same yields work.

This is because SharedTaskState::waitable_set starts as None and is only created in add_waitable. The waitable set is unwrapped unconditionally:

CallbackCode::Yield => {
    let set = state.shared.waitable_set.try_lock().unwrap();
    event = set.as_ref().unwrap().poll()
}

I don't know if there is a good rational for this, but perhaps the waitable_set should not be an Option or the arm should return (EVENT_NONE, 0, 0) instead of unwrapping.