Implement lazy cartesian product

[xgbuils]

Now, cartesian method does not allow to be used with infinite iterables. If cartesian returns an iterable of iterables, this method will be able to be lazy.

[xgbuils]

Thinking about it, cartesian method implementation is already lazy. This method allows a finite number of iterables. These iterables can be potentially infinite. When I thought in lazy cartesian method, I thought in a product of infinite iterables. However this thing is impossible to implement because we need to know if all of iterables are empty or not to check if the iterable result is empty or not.

Then, I just have to implement cartesian method that returns an Iterum instance of Iterum instances.

[xgbuils]

However, the current implementation of cartesian returns values in this order:

Iterum([1, 2]).cartesian([3, 4]) /* (
    (1 3)
    (1 4)
    (2 3)
    (2 4)
) */

For consistency, I prefer:

Iterum([1, 2]).cartesian([3, 4]) /* (
    (1 3)
    (2 3)
    (1 4)
    (2 4)
) */

Because there are other methods like permutations, power, combinations, variations that could be work with infinite iterables and numbers if the first value of iterables is that variates first.

[xgbuils]

I also going to change cartesian method name to product.