Word128

[andrewthad]

The Word128 data type from wide-word could stand to have a builder. Doing this is a little tricky. The naive approach (repeated division by 10) will not perform well since the implementation of division for Word128 is lengthy (I'll refer to this as "super divide"). I think the best strategy is to break the Word128 into three Word64s by using super divide twice. Then each of the three words can be handled the normal way.