Gateway: Fix serenity breaking changes (#173)

Fix issues caused by serenity-rs/serenity#2372 and serenity-rs/serenity#2380.

Additionally, this Boxes the TrySendError from Serenity at Clippy's behest (causing a ~330B Result type).

---------

Co-authored-by: Kyle Simpson <kyleandrew.simpson@gmail.com>

src/manager.rs

@@ -17,8 +17,7 @@ use once_cell::sync::OnceCell;
 use parking_lot::RwLock as PRwLock;
 #[cfg(feature = "serenity")]
 use serenity::{
-    client::bridge::voice::VoiceGatewayManager,
-    gateway::InterMessage,
+    gateway::{ShardRunnerMessage, VoiceGatewayManager},
     model::{
         id::{GuildId as SerenityGuild, UserId as SerenityUser},
         voice::VoiceState,
@@ -416,7 +415,7 @@ impl VoiceGatewayManager for Songbird {
         debug!("Songbird ({:?}) Initialised!", user_id);
     }
 
-    async fn register_shard(&self, shard_id: u32, sender: Sender<InterMessage>) {
+    async fn register_shard(&self, shard_id: u32, sender: Sender<ShardRunnerMessage>) {
         debug!(
             "Registering Serenity shard handle {} with Songbird",
             shard_id