Fix normalization of negative digits.
Manually unroll some maps into for loops. Fix some error types.
Add pow2 for quick 2^n. Copy better pow2 error-checking to pow as well.
Fix normalize's 0-dropping. Add pow() fast-paths. Let pow2() go higher.
Formatting tweaks - make fastpaths more consistent, move divmod error handling up to match other functions.
Make divmod accept large divisors. (Currently broken for negatives.)
Fix sign handling in add(), divmod(), and normalize().