message: Extract timer into variable so subscription doesn't get dropped.
diff: @@ -25,7 +25,8 @@ pub fn sleep(duration: Duration) {
  let expired = Cell::new(false);
  let mut with_callback = with_callback(|_, _| expired.set(true));
  - with_callback.init().unwrap().set_alarm(duration).unwrap();
  + let mut timer = with_callback.init().unwrap();
  + timer.set_alarm(duration).unwrap();
  syscalls::yieldk_for(|| expired.get());
  }
  