Fix clippy and enable clippy on CI

src/data/arena.rs

@@ -269,7 +269,7 @@ impl<T> Arena<T> {
                     self.free_list_head = next_free;
                     self.len += 1;
                     Some(Index {
-                        index: i as u32,
+                        index: i,
                         generation: self.generation,
                     })
                 }

src/data/pubsub.rs

@@ -106,8 +106,7 @@ impl<T> PubSub<T> {
     /// Read the i-th message not yet read by the given subsciber.
     pub fn read_ith(&self, sub: &Subscription<T>, i: usize) -> Option<&T> {
         let cursor = &self.cursors[sub.id as usize];
-        self.messages
-            .get(cursor.next(self.deleted_messages) as usize + i)
+        self.messages.get(cursor.next(self.deleted_messages) + i)
     }
 
     /// Get the messages not yet read by the given subscriber.