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.