Support passing values through the Mutex inside the parker

[james7132]

The caller of Unparker::unpark briefly has the lock on the Mutex before notifying the condvar. Would it be feasible to pass a Send value through the Mutex to the unparked thread for it to consume? Perhaps, say, a async_task runnable? If the threads already unparked or never was to begin with,unpark can just return back an error containing the value.

[notgull]

This might be a breaking change. However as this crate isn't directly re-exported by smol I would be fine with a break.

I guess this would have the same point as tagged events in event-listener. I mostly just want to keep the ergonomics of Parker intact so it can be used in, e.g., futures-lite and async-io.