registerAwaiter

    fn registerAwaiter(group: Group, awaiter: *Fiber) bool {
        assert(awaiter.status.queue_next == null);
        awaiter.status = .{ .awaiting_group = group };
        assert(@atomicRmw(
            Awaiter,
            group.awaiterPtr(),
            .Add,
            .{ .locked = false, .contended = false, .awaiter = .pack(awaiter) },
            .monotonic,
        ).awaiter == .null);
        return awaiter.cancel_status.changeAwaiting(.nothing, .group);
    }