Collect entropy bits from more dice sides

[philipmw]

Right now we collect entropy bits only from the first 2^n sides of dice. We discard rolls on sides above that.

For example, for six-sided dice, we collect entropy from sides 1--4 but discard rolls landing on 5 and 6.

This is wasteful and may be confusing to the user.

To collect entropy from the largest range of dice sides, we need to implement algorithm Q2 from this paper: http://markus-jakobsson.com/papers/jakobsson-ieeeit00.pdf