revset: remove the now unused _descgeneratorset class

revset: use _generatorset in _revancestors The _descgeneratorset class is going away.

revset: remove now unused class _ascgeneratorset

revset: use _generatorset directly in _revdescendant _ascgeneratorset is going away.

generatorset: move membership testing on ordered gen to the main class We are phasing out the ordered version of the class to simplify the code.

generatorset: make use of the new mechanism in the subclass Until we remove them, we use the new parameter of _generatorset to make sure the code is run.

generatorset: make it possible to use gen as fastasc or fastdesc We gain a parameter to inform that the generator is ascending or descending. If the generator is ordered, it is also used for the `fastasc` or `fastdesc` version. The _ascgeneratorset and _descgeneratorset class will be removed soon.

baseset: rely on the abstractsmartset implementation for filter

_orderedsetmixin: drop this now unused class All my friends are dead.

spanset: drop _orderedsetmixin inheritance The min/max method are as well provided by abstractsmartset.

orderedlazyset: drop this now unused class All my friends are dead.

_descendant: use filteredset instead of orderedlazyset The orderedlazyset class is going away. Filteredset gives the same service.

addset: use the base implementation for ascending and descending

addset: use base implementation for __filter__