The Python interpreter

The Python interpreter draft.  

Other comments:
- I've checked in tests, although they're not part of the 500 lines / not for inclusion in the book.  (They're not that interesting anyway - just verifying that little snippets of Python do the right thing.)
- It was less work than I thought it would be to adapt this from @nedbat's byterun, so we should make sure he gets appropriate credit.
Something that might be worth thinking about; it might be easier to explain and walkthrough call/return semantics, and then add a section later that deals with exception handling. I'll comment on this further when I hit the parts that discuss it.