Set value of derive statistic

We didn't have a `bws_derive_set` function before since derives are supposed to be counters that never decrease in value.  By only providing incrementors, we could guarantee that just using the C type system.  But, we already have a use case where we need to fill in a derive counter from a source that is itself always-increasing, and I don't really want to play arithmetic shenanigans just to get around the fact that we don't have a setter.  So now we do!