Make genRandArray accept 64 bit ints for the array length

[bartgol]

It is possible that the user tries to stress test some functionalities for very large arrays. In this case, we might need 64 bits for their size.

Note: I don't think this is very likely, but the mod is also very simple (make the fcn's take long long/std::int_64t instead of int, or template on the int type), so we might as well do it.