Allow World::entity family of functions to take multiple entities and get multiple references back

[ItsDoot]

Objective

Following the pattern established in #15593, we can reduce the API surface of World by providing a single function to grab both a singular entity reference, or multiple entity references.

Solution

The following functions can now also take multiple entity IDs and will return multiple entity references back:

• World::entity
• World::get_entity
• World::entity_mut
• World::get_entity_mut
• DeferredWorld::entity_mut
• DeferredWorld::get_entity_mut

If you pass in X, you receive Y:

• give a single Entity, receive a single EntityRef/EntityWorldMut (matches current behavior)
• give a [Entity; N]/&[Entity; N] (array), receive an equally-sized [EntityRef; N]/[EntityMut; N]
• give a &[Entity] (slice), receive a Vec<EntityRef>/Vec<EntityMut>
• give a &EntityHashSet, receive a EntityHashMap<EntityRef>/EntityHashMap<EntityMut>

Note that EntityWorldMut is only returned in the single-entity case, because having multiple at the same time would lead to UB. Also, DeferredWorld receives an EntityMut in the single-entity case because it does not allow structural access.

Testing

• Added doc-tests on World::entity, World::entity_mut, and DeferredWorld::entity_mut

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