feat: make VCsWatcher, a watch channel of VCs

src/command/leave.rs

@@ -55,10 +55,10 @@ pub fn get_user_and_guild_and_voice_channel_id(
 
     let guild_id = interaction.guild_id.context(NotInGuildSnafu)?;
 
-    let voice_channel_id = vcs
-        .with_guild(guild_id, |guild_vcs| {
-            guild_vcs.get_left_for(&bot_user_id).copied()
-        })
+    let &voice_channel_id = vcs
+        .get(&guild_id)
+        .context(BotNotInVCSnafu)?
+        .get_left_for(&bot_user_id)
         .context(BotNotInVCSnafu)?;
 
     Ok((user_id, guild_id, voice_channel_id))
@@ -80,11 +80,12 @@ fn get_guild_and_vc_error_to_embed(error: GetGuildAndVoiceChannelIdError) -> Emb
 
 #[tracing::instrument]
 pub async fn handle(state: State, interaction: Interaction) {
-    let (user_id, guild_id, voice_channel_id) = match get_user_and_guild_and_voice_channel_id(
+    let user_and_guild_and_voice_channel_id_res = get_user_and_guild_and_voice_channel_id(
         state.discord_user_id,
         &interaction,
-        &state.vcs,
-    ) {
+        &state.vcs_watcher.borrow(),
+    );
+    let (user_id, guild_id, voice_channel_id) = match user_and_guild_and_voice_channel_id_res {
         Ok((user_id, guild_id, voice_channel_id)) => (user_id, guild_id, voice_channel_id),
         Err(error) => {
             state