DuplexStream does not participate in coop

[seanmonstar]

The tokio::io::duplex() stream types do no participate in coop, so they can starve a task. This minimal example currently hangs:

use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::test]
async fn duplex_coop() {
    let (mut tx, mut rx) = tokio::io::duplex(1024 * 8);
    
    tokio::select! {
        biased;

        _ = async {
            loop {
                let buf = [3u8; 4096];
                let _ = tx.write_all(&buf).await;
                let mut buf = [0u8; 4096];
                let _ = rx.read(&mut buf).await;
            }
        } => {},
        _ = tokio::task::yield_now() => {}
    }
}

We should add coop checks in poll_read and poll_write, similar to what was done for #4291.

[GongLG]

Taking a look..