Derive Macro IntoR 

#[derive(IntoR)]

Derive IntoR for a single-field newtype: forward the Rust → R conversion to the inner type.

Generates a scalar IntoR impl that delegates to the inner type, plus an IntoRNewtype marker (powering the Option<T> / Vec<Option<T>> container blankets) and a concrete IntoRVecElement impl (powering Vec<T>). See #[derive(TryFromSexp)] for usage.

Do not derive both IntoR and MatchArg on the same type: both feed the single IntoR for Vec<T> blanket slot and would collide (E0119).