Remove _preorder as alias for dispatch and call dispatch directly.
Add an extra optional argument to walk()
XXX Also comment out some code that does debugging prints.
- fix tab space issues (SF patch #101167 by Neil Schemenauer)
- fix co_flags for classes to include CO_NEWLOCALS (SF patch #101145 by Neil)
- fix for merger of UNPACK_LIST and UNPACK_TUPLE into UNPACK_SEQUENCE,
(SF patch #101168 by, well, Neil :)
- Adjust bytecode MAGIC to current bytecode.
TODO: teach compile.py about list comprehensions.