Finish moving constant generation to Code.py

[dagss]

During my GSoC I started on refactoring the allocation of globals; however it is still using a backwards-compatible hybrid solution. This needs cleaning up.

Pyrex has some really good changes gone into it in this area (Greg can be a lot more confident about not breaking things so I suppose he just took it all the way at once); see the commits prior to and leading up to this one:

 http://hg.cython.org/pyrex/rev/da6e97bb7e6d

[dagss]

Duplicate of #144. (Closing the oldest one because the second on is better described.)