- Refactored code, to have type specific and generic binary search functions
  share signature. `get_(arg)binsearch_func` now handles finding the right
  function for all cases, making it transparent to `PyArray_SearchSorted`.
- Reordered types in `(arg)binsearch_map` to be sorted by type num. Since
  the function is searched using binary search, this is a requirement. Only
  `npy_half` was out of place. Added comment to remind future expansions.
- Corrected reference counting for `dtype` in `PyArray_SearchSorted`.
- The requirement for a coniguous `sorter` array had been dropped, but the
  stride for it was set to the element size. This created an opportunity for
  a very unlikely bug, that is now corrected. Added test to verify it.
- Removed unused includes from `binsearch.c.src` and `npy_binsearch.h.src`.