Function rsplitn 

pub fn rsplitn<H, P>(
    haystack: H,
    n: usize,
    needle: P,
) -> RSplitN<H, P::Searcher> 
where
    H: Haystack,
    P: Needle<H>,
    P::Searcher: ReverseSearcher<H::Target>,
    H::Target: Hay,