Macro impl_vec_option_coerce_into_r 

macro_rules! impl_vec_option_coerce_into_r {
    ($from:ty => $to:ty) => { ... };
}

Macro for Vec<Option<T>> where T coerces to a type with existing Option impl.

Delegates to the target type’s Vec<Option<$to>> impl (which itself uses alloc_r_vector).