A range pack: the packed stored codes and each range's filled OUTPUT length.
Stored codes of every range, at offsets[i] / perElement.
offsets[i] / perElement
Output elements each range actually fills (< counts[i] for a short range).
< counts[i]
True when every range filled its whole span (the pack has no gaps).
A range pack: the packed stored codes and each range's filled OUTPUT length.