Driver: Fix incorrect leave behaviour in Drop handler

Sending poison messages should suffice to kill the voice session: attempting to `.leave()`. Fixes #88.

This was tested using `cargo make ready` and the modified `serenity/voice/` example.

examples/serenity/voice/Cargo.toml

@@ -18,4 +18,4 @@ features = ["client", "standard_framework", "voice", "rustls_backend"]
 
 [dependencies.tokio]
 version = "1.0"
-features = ["macros", "rt-multi-thread"]
+features = ["macros", "rt-multi-thread", "signal"]

examples/serenity/voice/src/main.rs

@@ -63,7 +63,12 @@ async fn main() {
         .await
         .expect("Err creating client");
 
-    let _ = client.start().await.map_err(|why| println!("Client ended: {:?}", why));
+    tokio::spawn(async move {
+        let _ = client.start().await.map_err(|why| println!("Client ended: {:?}", why));
+    });
+    
+    tokio::signal::ctrl_c().await;
+    println!("Received Ctrl-C, shutting down.");
 }
 
 #[command]