WIP: add type specific binary search functions to speed up `searchsorted`

* Added API comment to `PyArray_FastSearchSorted`
* Added `bindigitize` https://github.com/numpy/numpy/pull/4184) for
  easier benchmarking
* No `fastsearchsorted` with `sorter` keyword argument
* No tests!!!
Merge branch 'fast_searchsorted' of https://github.com/jaimefrio/numpy into fast_searchsorted