Provide a safe abstraction for split access to entities and commands

[ItsDoot]

Objective

Currently experimenting with manually implementing Relationship/RelationshipTarget to support associated edge data, which means I need to replace the default hook implementations provided by those traits. However, copying them over for editing revealed that UnsafeWorldCell::get_raw_command_queue is pub(crate), and I would like to not have to clone the source collection, like the default impl. So instead, I've taken to providing a safe abstraction for being able to access entities and queue commands simultaneously.

Solution

Added World::entities_and_commands and DeferredWorld::entities_and_commands, which can be used like so:

let eid: Entity = /* ... */;
let (mut fetcher, mut commands) = world.entities_and_commands();
let emut = fetcher.get_mut(eid).unwrap();
commands.entity(eid).despawn();

Testing

• Added a new test for each of the added functions.