ENH: improve bool argmin performance

in glibc memchr using SSE2 is about 18 times faster than our current
code.
ENH: vectorize BOOL_argmax

using the same method as BOOL_logical_and
BUG: Fix np.insert for inserting a single item into a structured array

Note that there are some object array special cases because of allowing
multiple inserts. `np.array(..., dtype=object)` is not always clear.
Merge pull request #5004 from juliantaylor/argmin-performance

ENH: improve bool arg{min.max} performance
Merge pull request #5022 from seberg/structured-insert

BUG: Fix np.insert for inserting a single item into a structured array