WIP: searchsorted implementation

closes #6712

- [ ] `timedelta64` tests
- [ ] more edge case testing
- [ ] how should non-monotonic indexes be handled (currently raising a `ValueError`)?
- [ ] `datetime64` (and probably `timedelta64`) coercion needs work
- [ ] `side` argument testing
ahh, then you should catch that I think. The problem with ``self.index[sorted]`` is that if you have duplicates this will fail (test that). Maybe we should just fix take so it can take an out-of-bounds indexer.