Macro skeleton_impls

macro_rules! skeleton_impls {
    {
        struct $Skeleton:ident $ComputedSkeleton:ident {
            $(+ $mesh_bone:ident)*
            $($bone:ident)*
            $(::
            $($field:ident : $field_ty:ty),* $(,)?
            )?
        }
    } => { ... };
}