Round out Mut<T> api using lenses + auto-unwrapping

[TheRawMeatball]

Objective

• Allow going from Mut<LargeStruct> to Mut<Field>
• Simplify working with components made with the newtype pattern

Solution

Add lenses, which allow for getting a Mut<U> from a Mut<T> where T contains a U, and add a "default lens" option to the Component trait, allowing for automatic unwrapping of newtypes

Changelog

• Added Lens trait
• Added associated methods to Mut for lensing
• Added DefaultLens associated type to Component for automatic lensing
• Added Raw and RawMut query params which bypass automatic lensing

[alice-i-cecile]

The need for this sort of change makes me worried: I don't love that users will have to reason about this when trying to write generic systems.

[TheRawMeatball]

Maybe, but it's a fairly mindless fix - just use the Raw and RawMut wrappers for directly generic components. I think ergonomic wins for the more common cases outweigh this slight loss here.

[alice-i-cecile]

Interesting. My first impressions are:

• this needs way more docs, especially focused around explaining what a lens is and why users care. I think the most useful thing would be a series of progressively more advanced doc examples
• this seems like it's competing for design space with [Merged by Bors] - bevy_derive: Add derives for Deref and DerefMut #4328, and needs an explanation in the PR description of why it's better (and we should consider reverting [Merged by Bors] - bevy_derive: Add derives for Deref and DerefMut #4328 if we go this direction).
• this seems like it might be useful for enabling granular change detection, especially on things like Assets

[TheRawMeatball]

This is similar to #4328, but distinct in an important way: This method has benefits even when working with newtypes that wrap primitives. As for #4328, it can also be used for resources which could be its main selling point.

[alice-i-cecile]

As for #4328, it can also be used for resources which could be its main selling point.

This only holds until we have #[derive(Resource)] though 🧪

[alice-i-cecile]

Closed in favor of #6199.