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.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.