Optimize Iterator::count for event iterators (#7582)

# Objective

Related to #7530.

`EventReader` iterators currently use the default impl for `.count()`, which unnecessarily loops over all unread events.

# Solution

Add specialized impls that mark the `EventReader` as consumed and return the number of unread events.

crates/bevy_ecs/src/event.rs

@@ -378,6 +378,10 @@ impl<'a, E: Event> Iterator for ManualEventIterator<'a, E> {
         self.iter.next().map(|(event, _)| event)
     }
 
+    fn count(self) -> usize {
+        self.iter.count()
+    }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
@@ -445,6 +449,11 @@ impl<'a, E: Event> Iterator for ManualEventIteratorWithId<'a, E> {
         }
     }
 
+    fn count(self) -> usize {
+        self.reader.last_event_count += self.unread;
+        self.unread
+    }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.chain.size_hint()
     }