Expose query accesses

[joseph-gio]

Objective

It would be useful to be able to inspect a QueryState's accesses so we can detect when the data it accesses changes without having to iterate it. However there are two things preventing this:

• These accesses are unnecessarily encapsulated.
• Has<T> indirectly accesses T, but does not register it.

Solution

• Expose accesses and matches used by QueryState.
• Add the notion of "archetypal" accesses, which are not accessed directly, but whose presence in an archetype affects a query result.

[alice-i-cecile]

And so we inch closer to the access model laid out in bevyengine/rfcs#30 :D

I agree with this direction, and this PR is useful both as an ECS primitive and as something we can expose to users. I'd like to clean up the explanation and docs a bit further before merging.

Ideally we'd have module level documentation explaining the different kinds of access and how they relate to each other, but I won't block on that in this PR.

[alice-i-cecile]

Solid stuff. One small set of suggestions to clarify further, but I'm happy with this now.

[joseph-gio]

Ideally we'd have module level documentation explaining the different kinds of access and how they relate to each other, but I won't block on that in this PR.

I'll think on what I would write here, but I probably won't include anything in this PR since module-level documentation generally requires a lot more thought than method-level documentation.