Allow string iteration with a step !=1
Allow string iteration with a step !=1

Using a custom iterator, of course. As string are immutable in python,
the iterator is always a const_iterator.

hainm/pythran,pbrunet/pythran,artas360/pythran,pbrunet/pythran,pbrunet/pythran,artas360/pythran,serge-sans-paille/pythran,pombredanne/pythran,artas360/pythran,hainm/pythran,pombredanne/pythran,pombredanne/pythran,hainm/pythran,serge-sans-paille/pythran