Function intersperse_fold

fn intersperse_fold<I, B, F, G>(
    iter: I,
    init: B,
    f: F,
    separator: G,
    started: bool,
    next_item: Option<I::Item>,
) -> B
where
    I: Iterator,
    F: FnMut(B, I::Item) -> B,
    G: FnMut() -> I::Item,