Add iterator for unordered map data type

[marcauberer]

Add iterator for unordered map data type. This makes UnorderedMap<T, U> type compatible with foreach loops. So now this is possible:

UnorderedMap<int, int> map;
map.insert(1, 2);
map.insert(2, 3);
map.insert(3, 4);
map.insert(4, 5);
map.insert(5, 6);
foreach Pair<int&, int&> item : map {
    printf("%d: %d\n", item.getFirst(), item.getSecond());
}