WIP: Kill limit() heuristics

This is a continuation of #2000 

Todo:
- [ ] collect issues on google code (limit fails, but gruntz does not)
- [ ] fix limit(x**-pi, x, 0, dir='-').  Probably, add some stuff in heuristics()
- [ ] profile?  Our test suite seems to be not affected too much.
- [ ] rebase?