Use type aliases for Query item types in Query and QueryState methods

[alice-i-cecile]

Objective

• The Query and QueryState methods have a large number of <<Q as WorldQuery>::ReadOnlyFetch as Fetch<'w, 's>>::Item return types
• The unaliased types are mysterious and hard to read, especially for beginners

Solution

• Use the QueryItem type alias and a parallel ReadOnlyQueryItem alias to make this code easier to read and write.

Future Work

I think we should consider refactoring QueryIter to have an Item associated type and use these aliases there too. I'd prefer to keep this minimal though, so we can merge it ASAP and make #3333 easier.

[alice-i-cecile]

This comment was blatantly false, and seems to just be a mistake. It contradicts the safety comment, and doesn't seem to make sense with the API.

[bjorn3]

Does rustdoc show the type aliases or do they get normalized?

[alice-i-cecile]

This was a serious barrier to productivity and correctness in #3333.

[alice-i-cecile]

Does rustdoc show the type aliases or do they get normalized?

Normalized, unfortunately: rust-lang/rust#15823

[BoxyUwU]

This code exposes a large number of tricky lifetimes, which are prone to soundness problems

These lifetimes are still exposed and are still just as prone to soundness problems 🤷‍♀️ this seems like a nice win for readability but I don't think it really has anything to do with soundness or correctness of the ecs.

[alice-i-cecile]

Closing this out; not very useful until the rust bug is fixed.