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.
Before we jump into the implementation, it might be worth taking the `three` example and walking through a multistep example that starts with the "initial state" of the stack machine, and how the stacks and frames change throughout the execution of this function.