Add many_entities_mut function variants to DeferredWorld

[ItsDoot]

Objective

Closes #15038.

Solution

Added:

• DeferredWorld::get_many_entities_mut
• DeferredWorld::many_entities_mut
• DeferredWorld::get_many_entities_dynamic_mut
• DeferredWorld::get_many_entities_from_set_mut
• UnsafeWorldCell::get_entities (used by the above functions)
• UnsafeWorldCell::get_entities_dynamic (used by the above functions)

Changed:

• Made equivalent functions on World reuse the newly introduced methods on UnsafeWorldCell for their implementations.
• Fixed the safety comment on DeferredWorld::get_entity_mut.

Testing

• Added 3 tests for the World functions since they were previously missing.
• Added 3 tests for the DeferredWorld functions.

---

Migration Guide

• World::get_many_entities and World::get_many_entities_dynamic now return QueryEntityError instead of Entity in the Err path to match the signatures of their _mut counterparts.

[chescock]

It's nice to have a little more API consistency between the various World variants! I like how you were able to share more code between the mut and non-mut versions by putting common code in UnsafeWorldCell.

[chescock]

The _mut methods on World look the same as the ones on DeferredWorld. Could you share the implementation by doing something like DeferredWorld::from(self).get_many_entities_mut();? (Or maybe there is a simpler way to get from &mut World to DeferredWorld?)

[ItsDoot]

DeferredWorld::from(self).get_many_entities_mut();

This doesn't work unfortunately:

error[E0515]: cannot return value referencing temporary value
   --> crates\bevy_ecs\src\world\mod.rs:766:9
    |
766 |         DeferredWorld::from(self).get_many_entities_mut(entities)
    |         -------------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |         |
    |         returns a value referencing data owned by the current function
    |         temporary value created here

[chescock]

Oh, I see, &mut DeferredWorld is only a borrow of a borrow. To make that work, you'd need DeferredWorld.get_many_entities_mut() to take self by value so that it can return values with the 'w lifetime, but then you'd need to call .reborrow() almost every time you use it.

[pablo-lua]

Can we have an assertion to a error of non-existent entity? Entity::PLACEHOLDER is probably useful for this.

[ItsDoot]

Closed in favor of #15614