Provides AsyncWrite with flush

As per Tokio and others, this commit provides a `poll_flush` method on `AsyncWrite` so that a best-effort attempt at wakening once all bytes are flushed can be made.

examples/nrf/src/bin/buffered_uart.rs

@@ -61,5 +61,8 @@ async fn main(_spawner: Spawner, p: Peripherals) {
         info!("writing...");
         unwrap!(u.write_all(&buf).await);
         info!("write done");
+
+        // Wait until the bytes are actually finished being transmitted
+        unwrap!(u.flush().await);
     }
 }