Derive macro that generates any_kps() -> Vec<AKp> returning all field/variant keypaths as any keypaths.
Requires #[derive(Kp)] so the keypath accessor methods exist.
AKp type-erases both Root and Value, enabling heterogeneous collections of keypaths.
Derive macro that generates partial_kps() -> Vec<PKp<Self>> returning all field/variant keypaths.
Requires #[derive(Kp)] so the keypath accessor methods exist.