Function into_group_map_with_hasher 

pub fn into_group_map_with_hasher<I, K, V, S>(
    iter: I,
    hash_builder: S,
) -> HashMap<K, Vec<V>, S>
where
    I: Iterator<Item = (K, V)>,
    K: Hash + Eq,
    S: BuildHasher,

Return a HashMap of keys mapped to a list of their corresponding values.

See .into_group_map() for more information.