_descendants: directly use smartset As `addset` objects are proper smartset objects, we do not need to make any transformation of the result.

baseset: explicitly track order of the baseset A baseset starts without an explicit order. But as soon as a sort is requested, we simply register that the baseset has an order and use the ordered version of the list to behave accordingly. We will want to properly record the order at creation time in the future. This would unlock more optimisation and avoid some sorting.

baseset: fix isascending and isdescending We now have sufficient information to return the proper value there.

baseset: prepare lazy ordering in __iter__ We'll explicitly track the order of the baseset to take advantage of the ascending and descending lists during iteration.