Function fmt_elements_split_by_commas 

fn fmt_elements_split_by_commas<E, I, F>(
    iter: I,
    f: &mut Formatter<'_>,
    format_element: F,
) -> Result
where
    I: Iterator<Item = E>,
    F: Fn(E, &mut Formatter<'_>) -> Result,

helper formatting array elements with a comma and a space between them