Function convert_slot 

fn convert_slot<T: TryFromSexp>(
    prot: SEXP,
    at: R_xlen_t,
    name: &str,
    owner_type: &str,
) -> T
where
    <T as TryFromSexp>::Error: Display,

The element of prot at at, the slot of Sidecar field name, converted to T.