Re-implement the warnings module in C for speed and to reduce start up time.

I don't remember the exact state of this patch.  I'm sure it needs cleanup.  IIRC the only thing missing feature-wise was processing command line arguments.  Though I'm not entirely sure.  It's been a while since I did it.

I think I may have not used as many goto's in the code.  I'm also thinking I didn't like it as the error handling was too complex.  This definitely needs review.  If anyone wants to finish this off, go for it.  I'll probably return to it, but it won't be for a few weeks at the earliest.  It would probably be good to make comments to remind me of what needs to be done.

The new file should be Python/_warnings.c.  I couldn't decide whether to put it under Python/ or Modules/.  It seems some builtin modules are in both places.  Maybe we should determine where the appropriate place is and move them all there.

I couldn't figure out how to get svn to do a diff of a file that wasn't checked in.  I think I filtered out all the unrelated changes.