Make Parent and Children components immutable

[tim-blackbird]

Objective

Minimal changes to make the hierarchy components immutable.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[bushrat011899]

Just some documentation that needs to be deleted now that apply_or_insert doesn't panic, otherwise I think this is good to go! A follow-up that adds hooks for upholding the Parent/Children relationship invariants would be the next logical step.

[chescock]

The doc comments for get_mut_assume_mutable() give the safety invariant as `T` must be a mutable component, which this safety comment doesn't satisfy. If this is intended to be legal, then I think we need to update the documentation for that method.

I thought the plan was that immutable components could only be updated by re-inserting them, though, and that this was not intended to be legal.

[bushrat011899]

I think updating the safety requirement for the *_assume_mutable methods is appropriate here in the interim. The ideal solution would be to use swapping to actually preserve immutability and avoid the unsafe methods, but that's a little cumbersome at the moment.

[tim-blackbird]

I agree that the safety messages should be changed, it confused me too.

The ideal solution would be to use swapping

What does 'swapping' mean here?

[bushrat011899]

"swapping" as in temporarily replacing Children/Parent with a dummy value so you can just have ownership over the component while you're editing it. Basically core::mem::swap. As an alternative, #16668 will add a with_component method which does this for you without the temporary value by directly triggering hooks/observers.