Implement ReadOnlyQueryData for Parent and Children

[bushrat011899]

Objective

• Improve the ergonomics of Parent and Children in queries.
• Act on a conversation had on Discord with @ItsDoot

Solution

Add an implementation of ReadOnlyQueryData (and the prerequisite traits) for Parent and Children. This does not conflict with the derived implementations from Component, allowing the wrapping types to still be accessed via &T and &mut T as currently.

Testing

• CI passed locally.

---

Showcase

Before

fn foo(foo_query: Query<(&Foo, &Parent)>, bar_query: Query<&Bar>) {
    for (foo, parent) in foo_query.iter() {
        let parent = parent.get();
        let bar = bar_query.get(parent).unwrap();
        do_something(foo, bar);
    }
}

After

fn foo(foo_query: Query<(&Foo, Parent)>, bar_query: Query<&Bar>) {
    for (foo, parent) in foo_query.iter() {
        let bar = bar_query.get(parent).unwrap();
        do_something(foo, bar);
    }
}

As can be seen above, the change in end-user code is minimal, but positive. It's unfortunate that it requires an unsafe implementation since we're able to delegate all the actually unsafe operations to the known-safe derived implementation from Component. But I think it's worth it for this (mild) improvement.

[alice-i-cecile]

Hmm. I don't think I like this: it adds user-facing complexity and an inconsistent API for a pretty modest ergonomic gain.

[bushrat011899]

Totally reasonable, I'm not too confident this is something that would be popular enough to get merged. I just wanted to try it out since it was on my mind anyway 😁

[ItsDoot]

To clarify what I meant originally, I intended that it be taken a step further and remove the ability to use &Parent/&mut Parent/&Children/&mut Children, rather than just introducing the non-reference version. This is inevitable anyways once we introduce relations, as all hierarchy management will likely be done through commands instead (i.e. moving the sort functions currently on Children into a command).