# Objective

There are currently too many disparate ways to handle entity mapping,
especially after #17687. We now have MapEntities, VisitEntities,
VisitEntitiesMut, Component::visit_entities,
Component::visit_entities_mut.

Our only known use case at the moment for these is entity mapping. This
means we have significant consolidation potential.

Additionally, VisitEntitiesMut cannot be implemented for map-style
collections like HashSets, as you cant "just" mutate a `&mut Entity`.
Our current approach to Component mapping requires VisitEntitiesMut,
meaning this category of entity collection isn't mappable. `MapEntities`
is more generally applicable. Additionally, the _existence_ of the
blanket From impl on VisitEntitiesMut blocks us from implementing
MapEntities for HashSets (or any types we don't own), because the owner
could always add a conflicting impl in the future.

## Solution

Use `MapEntities` everywhere and remove all "visit entities" usages.

* Add `Component::map_entities`
* Remove `Component::visit_entities`, `Component::visit_entities_mut`,
`VisitEntities`, and `VisitEntitiesMut`
* Support deriving `Component::map_entities` in `#[derive(Coomponent)]`
* Add `#[derive(MapEntities)]`, and share logic with the
`Component::map_entities` derive.
* Add `ComponentCloneCtx::queue_deferred`, which is command-like logic
that runs immediately after normal clones. Reframe `FromWorld` fallback
logic in the "reflect clone" impl to use it. This cuts out a lot of
unnecessary work and I think justifies the existence of a pseudo-command
interface (given how niche, yet performance sensitive this is).

Note that we no longer auto-impl entity mapping for ` IntoIterator<Item
= &'a Entity>` types, as this would block our ability to implement cases
like `HashMap`. This means the onus is on us (or type authors) to add
explicit support for types that should be mappable.

Also note that the Component-related changes do not require a migration
guide as there hasn't been a release with them yet.

## Migration Guide

If you were previously implementing `VisitEntities` or
`VisitEntitiesMut` (likely via a derive), instead use `MapEntities`.
Those were almost certainly used in the context of Bevy Scenes or
reflection via `ReflectMapEntities`. If you have a case that uses
`VisitEntities` or `VisitEntitiesMut` directly, where `MapEntities` is
not a viable replacement, please let us know!

```rust
// before
#[derive(VisitEntities, VisitEntitiesMut)]
struct Inventory {
  items: Vec<Entity>,
  #[visit_entities(ignore)]
  label: String,
}

// after
#[derive(MapEntities)]
struct Inventory {
  #[entities]
  items: Vec<Entity>,
  label: String,
}
```