Problem 129 - Repunit divisibility (optimized)

 - Use modulo arithmetic for A(n)
 - R(k) = (10^k - 1) / 9
 - n divides R(k)
 - (10^k - 1) / 9 == 0 mod n
 - 10^k = 1 mod 9n
 - iteratively generate 10^k mod 9n, terminate when result is 1