Function unique_by_with_hasher 

pub fn unique_by_with_hasher<I, V, F, S>(
    iter: I,
    f: F,
    hash_builder: S,
) -> UniqueBy<I, V, F, S> 
where
    V: Eq + Hash,
    F: FnMut(&I::Item) -> V,
    I: Iterator,
    S: BuildHasher,

Create a new UniqueBy iterator.