refactor: Fix some pendantic clippy issues

cadency_core/src/client.rs

@@ -16,6 +16,7 @@ pub struct Cadency {
 }
 
 impl Cadency {
+    #[must_use]
     pub fn builder() -> CadencyBuilder {
         CadencyBuilder::default()
     }

cadency_core/src/command.rs

@@ -80,7 +80,7 @@ impl TypeMapKey for Commands {
 pub(crate) async fn setup_commands(ctx: &Context) -> Result<(), serenity::Error> {
     let commands = utils::get_commands(ctx).await;
     // No need to run this in parallel as serenity will enforce one-by-one execution
-    for command in commands.iter() {
+    for command in &commands {
         command.register(ctx).await?;
     }
     Ok(())

cadency_core/src/handler/command.rs

@@ -22,7 +22,7 @@ impl EventHandler for Handler {
 
         info!("⏳ Started to submit commands, please wait...");
         match setup_commands(&ctx).await {
-            Ok(_) => info!("✅ Application commands submitted"),
+            Ok(()) => info!("✅ Application commands submitted"),
             Err(err) => error!("❌ Failed to submit application commands: {:?}", err),
         };
     }

cadency_yt_playlist/src/ytdlp.rs

@@ -33,7 +33,7 @@ impl YtDlp {
         self.command
             .args(self.args.clone())
             .spawn()
-            .and_then(|child| child.wait_with_output())
+            .and_then(std::process::Child::wait_with_output)
     }
 }