Reference Harvey and Van Der Hoeven's multiplication algorithm

[jeffgerickson]

Update references to the fastest known integer multiplication algorithm to David Harvey and Joris Van Der Hoeven's O(n log n)-time algorithm, discovered in 2019:

• https://hal.archives-ouvertes.fr/hal-02070778
• https://web.maths.unsw.edu.au/~davidharvey/papers/nlogn/
• http://www.texmacs.org/joris/nlogn/nlogn-abs.html

[jeffgerickson]

Note that this result almost closes the complexity of integer multiplication, thanks to another recent result by Peyman Afshani andcoauthors, relating multiplication to network coding.

https://arxiv.org/abs/1902.10935

So it's not clear if I should continue describing the complexity of integer multiplication as an open problem. Yes, it is still an open problem, but it's arguably not an algorithmic open problem any more.

[jeffgerickson]

Done. Removed discussion of iterated logs. No longer describe integer multiplication as an open problem.