Remove child components during on_remove

[matthunz]

Currently WithChild and WithChildren only register an on_add hook. This is incredibly convenient for bundling up hierarchies, but feels like it's missing a way to automatically remove these child components.

An alternative design might be to also register child removal during on_remove. This may add overhead while despawning a hierarchy, but I think for most cases it would be preferred to remove the spawned child components when WithChild or WithChildren is removed.

let mut world = World::default();

let parent = world.spawn(WithChild::new((A, B(3)))).id();
// FIXME: this should not be needed!
world.flush();

assert!(!world.entity(parent).contains::<A>());
assert!(!world.entity(parent).contains::<B>());

let children = world.get::<Children>(parent).unwrap();
assert_eq!(children.len(), 1);

let child_entity = children[0];
assert_eq!(world.get::<A>(child_entity), Some(&A));
assert_eq!(world.get::<B>(child_entity), Some(&B(3)));

world.entity_mut(parent).remove::<WithChild<(A, B)>>();
// FIXME: this should not be needed!
world.flush();

assert!(world.get::<A>(child_entity).is_none());
assert!(world.get::<B>(child_entity).is_none());