feat(list/map): Added `list.clone` and `map.clone`

[giann]

closes #112

If user has to iterate and mutate list/map at the same time, it can use those functions:

[int] values = [1, 2, 3];

foreach (int i, int value in values.clone()) {
    if (value % 2 == 0) {
        values.remove(i);
    }
}