Kill limit() heuristics

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