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.
The idea of a 1500-line long switch statement is going to be contrary to what some readers might have learned is a good idea. Why is it a good or practical solution to this problem?