Implement Clone for Fetches (#2641)

# Objective

This:

```rust
use bevy::prelude::*;

fn main() {
    App::new()
    .add_system(test)
    .run();
}

fn test(entities: Query<Entity>) {
    let mut combinations = entities.iter_combinations_mut();
    while let Some([e1, e2]) = combinations.fetch_next() {    
        dbg!(e1);
    }
}
```

fails with the message "the trait bound `bevy::ecs::query::EntityFetch: std::clone::Clone` is not satisfied". 


## Solution

It works after adding the naive clone implementation to EntityFetch. I'm not super familiar with ECS internals, so I'd appreciate input on this.

Author: msvbg

crates/bevy_ecs/src/query/fetch.rs

@@ -137,6 +137,7 @@ impl WorldQuery for Entity {
 }
 
 /// The [`Fetch`] of [`Entity`].
+#[derive(Clone)]
 pub struct EntityFetch {
     entities: *const Entity,
 }
@@ -573,6 +574,7 @@ impl<T: WorldQuery> WorldQuery for Option<T> {
 }
 
 /// The [`Fetch`] of `Option<T>`.
+#[derive(Clone)]
 pub struct OptionFetch<T> {
     fetch: T,
     matches: bool,
@@ -807,6 +809,21 @@ pub struct ChangeTrackersFetch<T> {
     change_tick: u32,
 }
 
+impl<T> Clone for ChangeTrackersFetch<T> {
+    fn clone(&self) -> Self {
+        Self {
+            storage_type: self.storage_type,
+            table_ticks: self.table_ticks,
+            entity_table_rows: self.entity_table_rows,
+            entities: self.entities,
+            sparse_set: self.sparse_set,
+            marker: self.marker,
+            last_change_tick: self.last_change_tick,
+            change_tick: self.change_tick,
+        }
+    }
+}
+
 /// SAFETY: access is read only
 unsafe impl<T> ReadOnlyFetch for ChangeTrackersFetch<T> {}