Minor fix for adix/bist.nim

Add promotion from SomeInteger to int, a need discovered while testing/timing BISTs as a top-k alternative. (Spoiler alert - need 2 or more quantiles to improve enough upon topk batch-quick-select, but then again (median,interquartile range) is "a thing" and is also 3 quantiles.)

Also added bist.clear to zero out counters with the same re-used memory.