Added function xgcd (extended-gcd) + unit tests.

The implementation is not copied from somewhere, I based it on the Wiki article, which I placed as an `@see` in the doc-comment.

I did "borrow" some unit tests from another math-library, which I mentioned in the comments of the unit tests.

I didn't add support arrays and/or matrices, since that didn't seem logical to me given that `xgcd` returns and array holding 3 values.

Feel free to rename `xgcd` to `egcd`, of course.