implement MapEntities for higher-order types (#19071)

# Objective

With the current `MapEntities` `impl`s, it is not possible to derive
things like this:

```rust
#[derive(Component)]
pub struct Inventory {
  #[entities]
  slots: Vec<Option<Entity>>,
}
```

This is because `MapEntities` is only implemented for `Vec<Entity>` &
`Option<Entity>`, and not arbitrary combinations of those.

It would be nice to also support those types.

## Solution

I replaced the `impl`s of the following types

- `Option<Entity>`: replaced with `Option<T>` 
- `Vec<Entity>`: replaced with `Vec<T>`
- `HashSet<Entity, S>`: replaced with `HashSet<T, S>`
- `T` also had to be `Eq + core:#️⃣:Hash` here. **Not sure if this is
too restrictive?**
- `IndexSet<Entity, S>`: replaced with `IndexSet <T, S>`
- `T` also had to be `Eq + core:#️⃣:Hash` here. **Not sure if this is
too restrictive?**
- `BTreeSet<Entity>`: replaced with `BTreeSet<T>`
- `VecDeque<Entity>`: replaced with `VecDeque<T>`
- `SmallVec<A: smallvec::Array<Item = Entity>>`: replaced with
`SmallVec<A: smallvec::Array<Item = T>>`

(in all of the above, `T` is a generic type that implements
`MapEntities` (`Entity` being one of them).)

## Testing

I did not test any of this, but extended the `Component::map_entities`
doctest with an example usage of the newly supported types.

---

## Showcase

With these changes, this is now possible:

```rust
#[derive(Component)]
pub struct Inventory {
  #[entities]
  slots: Vec<Option<Entity>>,
}
```

crates/bevy_ecs/src/component.rs

@@ -574,6 +574,17 @@ pub trait Component: Send + Sync + 'static {
     /// ```
     ///
     /// Fields with `#[entities]` must implement [`MapEntities`](crate::entity::MapEntities).
+    ///
+    /// Bevy provides various implementations of [`MapEntities`](crate::entity::MapEntities), so that arbitrary combinations like these are supported with `#[entities]`:
+    ///
+    /// ```rust
+    /// # use bevy_ecs::{component::Component, entity::Entity};
+    /// #[derive(Component)]
+    /// struct Inventory {
+    ///     #[entities]
+    ///     items: Vec<Option<Entity>>
+    /// }
+    /// ```
     #[inline]
     fn map_entities<E: EntityMapper>(_this: &mut Self, _mapper: &mut E) {}
 }

crates/bevy_ecs/src/entity/map_entities.rs

@@ -65,25 +65,38 @@ impl MapEntities for Entity {
     }
 }
 
-impl MapEntities for Option<Entity> {
+impl<T: MapEntities> MapEntities for Option<T> {
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
-        if let Some(entity) = self {
+        if let Some(entities) = self {
-            *entity = entity_mapper.get_mapped(*entity);
+            entities.map_entities(entity_mapper);
         }
     }
 }
 
-impl<S: BuildHasher + Default> MapEntities for HashSet<Entity, S> {
+impl<T: MapEntities + Eq + core::hash::Hash, S: BuildHasher + Default> MapEntities
+    for HashSet<T, S>
+{
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
-        *self = self.drain().map(|e| entity_mapper.get_mapped(e)).collect();
+        *self = self
+            .drain()
+            .map(|mut entities| {
+                entities.map_entities(entity_mapper);
+                entities
+            })
+            .collect();
     }
 }
 
-impl<S: BuildHasher + Default> MapEntities for IndexSet<Entity, S> {
+impl<T: MapEntities + Eq + core::hash::Hash, S: BuildHasher + Default> MapEntities
+    for IndexSet<T, S>
+{
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
         *self = self
             .drain(..)
-            .map(|e| entity_mapper.get_mapped(e))
+            .map(|mut entities| {
+                entities.map_entities(entity_mapper);
+                entities
+            })
             .collect();
     }
 }
@@ -97,35 +110,38 @@ impl MapEntities for EntityIndexSet {
     }
 }
 
-impl MapEntities for BTreeSet<Entity> {
+impl<T: MapEntities + Ord> MapEntities for BTreeSet<T> {
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
         *self = mem::take(self)
             .into_iter()
-            .map(|e| entity_mapper.get_mapped(e))
+            .map(|mut entities| {
+                entities.map_entities(entity_mapper);
+                entities
+            })
             .collect();
     }
 }
 
-impl MapEntities for Vec<Entity> {
+impl<T: MapEntities> MapEntities for Vec<T> {
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
-        for entity in self.iter_mut() {
+        for entities in self.iter_mut() {
-            *entity = entity_mapper.get_mapped(*entity);
+            entities.map_entities(entity_mapper);
         }
     }
 }
 
-impl MapEntities for VecDeque<Entity> {
+impl<T: MapEntities> MapEntities for VecDeque<T> {
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
-        for entity in self.iter_mut() {
+        for entities in self.iter_mut() {
-            *entity = entity_mapper.get_mapped(*entity);
+            entities.map_entities(entity_mapper);
         }
     }
 }
 
-impl<A: smallvec::Array<Item = Entity>> MapEntities for SmallVec<A> {
+impl<T: MapEntities, A: smallvec::Array<Item = T>> MapEntities for SmallVec<A> {
     fn map_entities<E: EntityMapper>(&mut self, entity_mapper: &mut E) {
-        for entity in self.iter_mut() {
+        for entities in self.iter_mut() {
-            *entity = entity_mapper.get_mapped(*entity);
+            entities.map_entities(entity_mapper);
         }
     }
 }