Convert to base 10 is significantly slower than similar libraries

[HKalbasi]

Other libraries like GMP use a divide and conquer algorithm which find a power of 10 with size near half of the digits of the number, use that as the base and convert the original number to a 2 digit number in that base, convert each digit recursively to base 10 and finally concat the results. Assuming that we have a O(nlogn) multiplication routine, this algorithm would be O(nlog^2 n) and would be faster than the current O(n sqrt n) algorithm.

[cuviper]

The current algorithm came in #216, and I mentioned then:

I honestly expect there's still room for improvement,

Help is welcome!

[HKalbasi]

I'm interested in helping, num-bigint is the default choice for arbitrary precision integers in Rust so I think it is important to make it fast.

The first step is to merge #282 or something like that since the above algorithm requires a fast multiplication algorithm.