Function list_to_vec_of_maps 

fn list_to_vec_of_maps<M>(sexp: SEXP) -> Result<Vec<M>, SexpError>
where
    M: TryFromSexp,
    M::Error: Into<SexpError>,

Helper to convert R list (VECSXP) to Vec<M> where each element is converted via M: TryFromSexp.