Function hashmap_union::union_of

pub fn union_of<K, V, F>(
    one: &HashMap<K, V>, 
    two: &HashMap<K, V>, 
    f: F
) -> HashMap<K, V> where
    K: Hash + Eq + Clone,
    V: Clone,
    F: Fn(V, V) -> V, 

Merges two hashmaps, using f where the keys exist in both hashmaps

• one: The first hashmap
• two: The second hashmap
• f: What to do when the key exists in both hashmaps
• Returns: The union of one and two