Add clone_components and move_components (and variants) to EntityWorldMut and EntityCommands

[JaySpruce]

Objective

Thanks to @eugineerd's work on entity cloning (#16132), we now have a robust way to copy components between entities. We can leverage this to implement some useful methods that would have been more complicated before.

Closes #15350.

Solution

Added the following methods to EntityWorldMut and EntityCommands:

• clone_components
• clone_components_with_requires
• clone_components_by_id
• move_components
• move_components_with_requires
• move_components_by_id

Full syntax:

• clone_components::<B: Bundle>(target: Entity)
• clone_components_with_requires::<B: Bundle>(target: Entity)
• clone_components_by_id(target: Entity, ids: impl IntoIterator<Item = ComponentId>)
• move_components::<B: Bundle>(target: Entity)
• move_components_with_requires::<B: Bundle>(target: Entity)
• move_components_by_id(target: Entity, ids: impl IntoIterator<Item = ComponentId> + Clone)

Also added clone_and_spawn and clone_and_spawn_with to EntityWorldMut (EntityCommands already had them).

Showcase

assert_eq!(world.entity(entity_a).get::<B>(), Some(&B));
assert_eq!(world.entity(entity_b).get::<B>(), None);
world.entity_mut(entity_a).clone_components::<B>(entity_b);
assert_eq!(world.entity(entity_a).get::<B>(), Some(&B));
assert_eq!(world.entity(entity_b).get::<B>(), Some(&B));

assert_eq!(world.entity(entity_a).get::<C>(), Some(&C(5)));
assert_eq!(world.entity(entity_b).get::<C>(), None);
world.entity_mut(entity_a).move_components::<C>(entity_b);
assert_eq!(world.entity(entity_a).get::<C>(), None);
assert_eq!(world.entity(entity_b).get::<C>(), Some(&C(5)));

Question

Should there be clone_components_by_id_with_requires and move_components_by_id_with_requires? They should be simple to add, but it doesn't look like other methods have them (there's remove_by_id and remove_with_requires, but no remove_by_id_with_requires).

Also, I'm not super familiar with the usage of dynamic components, so I'm less confident on the implementation there.