Consider using somebody else's U256 code

[benwr]

My U256 code is the first thing I thought of doing. i.e. It uses the elementary school multiplication algorithm. Which probably means that it's nowhere near as fast as it could be. Maybe I should look for someone else's implementation.

[benwr]

I tried to replace the U256 entirely with pure-u128 logic. It worked but was twice as slow. That experience, plus googling around for multiplication algorithms and not finding anything that looks substantially better than this has changed my mind: I now think this is basically fine.