Make PhysicsLayer require a #[default] variant

[Jondolf]

Objective

#476 changed collision layers to reserve the first bit for a default layer. However, for enums implementing PhysicsLayer, the default layer is rather implicit and potentially footgunny.

The default layer should ideally be more explicit.

Solution

Change PhysicsLayer to require Default to be implemented. The proc macro orders the variants such that the layer set as the default is always the first bit.

Note that Default must currently be derived instead of implemented manually, as the macro can only access the default variant if the #[default] attribute exists. An error is emitted if #[default] does not exist. If someone knows a way to make it work with the manual impl, let me know or consider opening a PR!

I also added documentation for the PhysicsLayer macro, and improved error handling.

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Migration Guide

Enums that derive PhysicsLayer must now also derive Default and specify a default layer using the #[default] attribute.

Before:

#[derive(PhysicsLayer)]
enum GameLayer {
    Player,
    Enemy,
    Ground,
}

After:

#[derive(PhysicsLayer, Default)]
enum GameLayer {
    #[default]
    Default, // The name doesn't matter, but Default is used here for clarity
    Player,
    Enemy,
    Ground,
}

The default layer always has the bit value 0b0001 regardless of its order relative to the other variants.