Change WebSocket API to use an extractor (#121)

Fixes https://github.com/tokio-rs/axum/issues/111

Example usage:

```rust
use axum::{
    prelude::*,
    extract::ws::{WebSocketUpgrade, WebSocket},
    response::IntoResponse,
};

let app = route("/ws", get(handler));

async fn handler(ws: WebSocketUpgrade) -> impl IntoResponse {
    ws.on_upgrade(handle_socket)
}

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = if let Ok(msg) = msg {
            msg
        } else {
            // client disconnected
            return;
        };

        if socket.send(msg).await.is_err() {
            // client disconnected
            return;
        }
    }
}
```

src/lib.rs

@@ -727,10 +727,6 @@ pub mod routing;
 pub mod service;
 pub mod sse;
 
-#[cfg(feature = "ws")]
-#[cfg_attr(docsrs, doc(cfg(feature = "ws")))]
-pub mod ws;
-
 #[cfg(test)]
 mod tests;