add tp_nextiter implementation for our iterators

[undingen]

                                7eec7fa2e53c908a06:  5c60ed7c85337bb516:
           django_template3.py             2.7s (4)             2.7s (4)  -0.6%
                 pyxl_bench.py             2.2s (4)             2.2s (4)  -1.3%
     sqlalchemy_imperative2.py             2.6s (4)             2.6s (4)  -1.4%
                       geomean                 2.5s                 2.5s  -1.1%

[undingen]

I'm not sure if we should not remove our __hasnext__ implementation because I think it doesn't give us as any advantage over tp_iternext which can just return NULL when it encountered the end!??

[kmod]

nice :)