Fixes TheAlgorithms#1701
The old algorithm used recursion which takes O(n) time to run, being
n the exponent.
This new algorithm uses the paradigm of `divide and conquer` with a time
complexity of O(log n) and space complexity of O(1).
This is a reimplementation of https://www.geeksforgeeks.org/write-a-c-program-to-calculate-powxn/ but this will only work with positive exponent since the signature of the function only allow natural numbers.