Rework class hierarchy in intrisic.py
Rework class hierarchy in intrisic.py

So that function intrinsic can also have a combiner.

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