Give change_detection its own folder and split into multiple files

[JaySpruce]

Objective

change_detection.rs is a bit big at ~1900 lines and its contents are pretty varied, so it ought to be split up.

Solution

Split change_detection.rs into 4 files:

• mod.rs: Re-exports, some constants, testing
• params.rs: Change-detecting system/query parameters like Ref and Res
• traits.rs: The DetectChanges and DetectChangesMut traits and the macros used internally to implement those traits (and some others) for the above parameters
• maybe_location.rs: MaybeLocation and its impls

This shouldn't change anything publicly, since everything's still in the same module.

I'd also like to move component/tick.rs into this module, but that would break stuff, so that'll be its own PR.

[alice-i-cecile]

I'm still not a fan of the name of this module, but that should be done later. Definitely think this is overdue for some reorganization though.

[JaySpruce]

What are you thinking for the name?

[alice-i-cecile]

What are you thinking for the name?

I'm not sure 🤔 It just feels really weird to have e.g. Mut inside of a module caused change_detection to me: it feels incidental.