- Modified generic searches to take a PyArrayObject and use its PyArray_DESCR
  to fetch a compare function, rather than the global state function that
  the sort functions take.
- Added get_genericbinsearch_func and get_genericargbinsearch_func to
  npy_binsearch.h.src, to get a function pointer to the right generic type
  search function. This makes the code in item_selection.c easier to follow,
  I think.
- Littered PyArray_SearchSorted with comments with my ref counts for dtype,
  hopefully I got it right this time.