feat: update to songbird 0.6, make joining calls work by spawning everything as required in songbird's documentation

src/command/mod.rs

@@ -23,12 +23,12 @@ pub struct State {
 }
 
 type Return = ();
-type BoxedHandler = Box<dyn Fn(State, Interaction) -> BoxFuture<'static, Return>>;
+type BoxedHandler = Box<dyn Send + Sync + Fn(State, Interaction) -> BoxFuture<'static, Return>>;
 
 fn box_handler<Handler, Fut>(handler: Handler) -> BoxedHandler
 where
     Fut: Future<Output = Return> + Send + 'static,
-    Handler: Fn(State, Interaction) -> Fut + 'static,
+    Handler: Send + Sync + Fn(State, Interaction) -> Fut + 'static,
 {
     Box::new(move |state, interaction| Box::pin(handler(state, interaction)))
 }
@@ -50,7 +50,7 @@ impl Router {
     fn add_route<Fut, Handler>(&mut self, name: &str, handler: Handler)
     where
         Fut: Future<Output = Return> + Send + 'static,
-        Handler: Fn(State, Interaction) -> Fut + 'static,
+        Handler: Send + Sync + Fn(State, Interaction) -> Fut + 'static,
     {
         self.add_route_already_boxed(name, box_handler(handler));
     }