Function replacen_with 

pub fn replacen_with<H, P, F, W>(
    src: H,
    from: P,
    replacer: F,
    n: usize,
    writer: W,
)
where
    H: Haystack,
    P: Needle<H>,
    F: FnMut(H) -> H,
    W: FnMut(H),
    H::Target: Hay,