Implement faster multiplication algorithm

[isaacholt100]

Currently, the multiplication algorithm for integers is long multiplication, which is slow for large numbers (O(n^2)). For integers of size beyond a given threshold, faster algorithms should be used such as Karatsuba, Toom-Cook, and maybe also Schonage-Strassen. This will also allow a divide and conquer division to be used which will have the same time complexity as the multiplication algorithm.