`PhysicsLayer` derive macro appears to expect undocumented cargo features to be present

[ilyvion]

Looking at the docs for CollisionLayers, it has this example code:

#[derive(PhysicsLayer, Default)]
enum GameLayer {
    #[default]
    Default, // Layer 0 - the default layer that objects are assigned to
    Player,  // Layer 1
    Enemy,   // Layer 2
    Ground,  // Layer 3
}

This PhysicsLayer derive macro produces the following output:

#[cfg(feature = "2d")]
use avian2d::prelude::PhysicsLayer;
#[cfg(feature = "3d")]
use avian3d::prelude::PhysicsLayer;
impl PhysicsLayer for GameLayer {
    fn all_bits() -> u32 {
        15u32
    }
    fn to_bits(&self) -> u32 {
        match self {
            GameLayer::Default => 1u32,
            GameLayer::Player => 2u32,
            GameLayer::Enemy => 4u32,
            GameLayer::Terrain => 8u32,
        }
    }
}

which in turn produces the following compiler warning:

warning: unexpected `cfg` condition value: `2d`
   --> src\game.rs:436:10
    |
436 | #[derive(PhysicsLayer, Default)]
    |          ^^^^^^^^^^^^
    |
    = note: expected values for `feature` are: `one-of-my-features`
    = help: consider adding `2d` as a feature in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
    = note: this warning originates in the derive macro `PhysicsLayer` (in Nightly builds, run with -Z macro-backtrace for more info)

I feel like it has to be a bug that the macro expects there to be two features in any given crate they're used in. At least I can't find it documented anywhere that it expects this.

[Jondolf]

The warning should be fixed by #601. I've already published a 0.2.1 patch for avian_derive with those changes, so running cargo update for apps using Avian 0.2 should be enough to fix it.