sync: support mpsc send with &self (#2861)

Updates the mpsc channel to use the intrusive waker based sempahore.
This enables using `Sender` with `&self`.

Instead of using `Sender::poll_ready` to ensure capacity and updating
the `Sender` state, `async fn Sender::reserve()` is added. This function
returns a `Permit` value representing the reserved capacity.

Fixes: #2637
Refs: #2718 (intrusive waiters)

tokio-test/src/io.rs

@@ -200,7 +200,9 @@ impl Inner {
     }
 
     fn poll_action(&mut self, cx: &mut task::Context<'_>) -> Poll<Option<Action>> {
-        self.rx.poll_recv(cx)
+        use futures_core::stream::Stream;
+
+        Pin::new(&mut self.rx).poll_next(cx)
     }
 
     fn read(&mut self, dst: &mut ReadBuf<'_>) -> io::Result<()> {