Function fold 

fn fold<'a, I: RunEndIndexType, V: ArrowNumericType, F, E>(
    array: TypedRunArray<'a, I, PrimitiveArray<V>>,
    f: F,
) -> Result<Option<V::Native>, E>
where
    F: FnMut(V::Native, V::Native, usize) -> Result<V::Native, E>,

Folds over the values in a run-end-encoded array.