Implement extended_gcd_by_iter

[Nugine]

Another implemention of exgcd

[EbTech]

Thanks for the contribution!

I prefer not to merge in duplicate functionality, though I might be persuaded otherwise. As for why recursive vs iterative, all else being equal I appreciate the stack frame saving benefit of iteration. However, in this case the recursion is relatively shallow, while the iterative solution costs us in terms of code size and number of mutable variables.

[Nugine]

Reference:
http://anh.cs.luc.edu/331/notes/xgcd.pdf
http://math.cmu.edu/~bkell/21110-2010s/extended-euclidean.html

The iterative version is based on a algorithm different from the recursive version. For the purpose of education, I prefer to maintain both of them.
In fact, The two versions have the same time complexity. Iteration does save time and memory, although the improvement is not obvious.