WIP: searchsorted implementation

closes #6712

we've decided to simply return `self.values.searchsorted`, since that's what was happening before, and because dealing with non-monotonic indices is a PITA and yields marginal benefit when you can just use `s.iloc[s.searchsorted(...)]`


- [ ] `timedelta64` tests
- [ ] more edge case testing
- [x] ~~how should non-monotonic indexes be handled (currently raising a `ValueError`)?~~
- [x] ~~`datetime64` (and probably `timedelta64`) coercion needs work~~
- [ ] `side` argument testing
- [ ] documentation
    - [ ] docstring
    - [ ] add to api.rst
    - [ ] add to basics.rst/indexing.rst
@JanSchulz let's add a ``raise NotImplementedError`` for ``Categorical``