Create better BigDecimal type

[non]

This is somewhat predicated on #226. If we had a faster big integer type, we could also build a better decimal type.

One mistakes to avoid repeating: do not store math contexts in particular instances. If someone is interested in this they should be experienced enough with numerical analysis to be sure they get the correct rounding/approximation results for decimal math.

This task could also involve creating base-2 fractional type rather than base-10, if that seemed better.