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.
I think the concepts of 'stack machine' vs 'register machine' require a bit more explanation, as probably both terms will be unfamiliar to many readers and so they won't have the context to compare them against one another.