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 personally think the statement that we can "fall back on Python's object system" needs some unpacking. Whenever we explore programs that are executing programs, the lines between the two can easily become blurred to the reader, so it's better to make those boundaries as explicit as possible IMO :)  

It might be better to hold off on mentioning this altogether until it becomes immediately relevant.