builder: assert nonnegative values

[yurivish]

https://github.com/iopsystems/h2-histogram-js/blob/main/src/index.js#L280

this would serve as a sanity check for subtraction of rolling histograms.

(and in fact, we will need to be exceedingly careful about exposing rolling histograms to the front-end since in their default representation they rely on u64 overflow behavior which means that deltas must be taken in that representation, with the result represented as a u64 or otherwise handling the overflow with a mod operation.)