Add helper methods to dynamically access relationships

[cBournhonesque]

Objective

#21601 introduced helpers to dynamically access relationship values
The RelationshipAccessor can be hard to use manually as it requires manipulating pointers; we can provide helper functions in UnsafeEntityCell and friends to access those.

[Victoronz]

Is there a reason why these functions return Box<dyn Iterator> instead of impl Iterator?
Also I am curious how it would error if the '_ are removed, given how use<'a> became a thing recently.

[cBournhonesque]

Is there a reason why these functions return Box<dyn Iterator> instead of impl Iterator? Also I am curious how it would error if the '_ are removed, given how use<'a> became a thing recently.

You're right, impl Iterator is much better! will change

[chescock]

Why can't this be self.as_readonly().get_relationship_targets_by_id(relationship_target_id), like get_relationship_by_id is? ... Oh, I see, that would borrow from a temporary. Wait, then how do we do that elsewhere? ... Hmm, I think some of the lifetimes are wrong here.

The read-only methods on mutable EntityRefs need to return things that borrow from '_ instead of 'w, since otherwise you could do

let borrows_r1 = filtered_entity.get_relationship_targets_by_id(r1);
let borrows_r1_mutably = filtered_entity.get_mut_by_id(r1);
// Both variables are alive here, because the lifetime in `borrows_r1`
// wasn't tied to the borrow of `filtered_entity`!

(If we want methods that return 'w, they need to consume self, like into_mut() does. We don't need those here, though, since they're uncommon and users can build them out of entity_ref.into_readonly().get_relationship_targets_by_id().)

So, I think this should be:

Suggested change

	) -> Option<impl Iterator<Item = Entity> + use<'w>> {
	self.access
	.has_component_read(relationship_target_id)
	// SAFETY: We have read access
	.then(|| unsafe {
	self.entity
	.get_relationship_targets_by_id(relationship_target_id)
	})
	.flatten()
	) -> Option<impl Iterator<Item = Entity> + use<'_>> {
	self.as_readonly().get_relationship_targets_by_id(relationship_target_id)

[chescock]

In contrast, EntityRef::get_relationship_targets_by_id can use 'w, because EntityRef knows nothing else can have mutable access for the rest of the 'w lifetime. So this can be:

Suggested change

	) -> Option<impl Iterator<Item = Entity> + use<'_>> {
	) -> Option<impl Iterator<Item = Entity> + use<'w>> {

[chescock]

And I'd recommend writing these in terms of as_readonly(), like we do with EntityMut::get(), since that means we can write them with safe code and not have to check as many lifetimes manually:

Suggested change

	// SAFETY: We have read-only access to all components of this entity.
	unsafe { self.cell.get_relationship_by_id(relationship_id) }
	self.as_readonly().get_relationship_by_id(relationship_id)

[chescock]

Suggested change

	// SAFETY: We have read-only access to all components of this entity.
	unsafe {
	self.cell
	.get_relationship_targets_by_id(relationship_target_id)
	}
	self.as_readonly().get_relationship_targets_by_id(relationship_target_id)