net: use &self with TcpListener::accept (#2919)

Uses the infrastructure added by #2828 to enable switching
`TcpListener::accept` to use `&self`.

This also switches `poll_accept` to use `&self`. While doing introduces
a hazard, `poll_*` style functions are considered low-level. Most users
will use the `async fn` variants which are more misuse-resistant.

TcpListener::incoming() is temporarily removed as it has the same
problem as `TcpSocket::by_ref()` and will be implemented later.

examples/chat.rs

@@ -77,7 +77,7 @@ async fn main() -> Result<(), Box<dyn Error>> {
     // Bind a TCP listener to the socket address.
     //
     // Note that this is the Tokio TcpListener, which is fully async.
-    let mut listener = TcpListener::bind(&addr).await?;
+    let listener = TcpListener::bind(&addr).await?;
 
     tracing::info!("server running on {}", addr);