merged trunk to ast-branch -- line numbers working

I did some work over the last couple of weeks to get line numbers
working on the ast branch.  If you get a traceback, it should not have
meaningful line numbers.

You've probably noticed that I updated the branch from the trunk in
order to get there.  This wasn't completely necessary, but was the first
way I thought of to get the interpreter to use co_lnotab instead of
SET_LINENO instructions.

There's now even more code that gets run when the interpreter starts
up.  warnings.py was one that caused problems.  There are some problems
with the code generated.  I haven't tracked down exactly what goes
wrong, but I hope the line numbers help.

Jeremy