ENH: speed-up minimum, maximum and xor for bool dtype
Replaced the operations for the above ufuncs on bools, by simpler
single instructions.

On random inputs `np.minimum` and `np.maximum` run about 25% faster,
and `np.logical_xor` and `np.bitwise_xor` almost 3x faster.
Merge pull request #4740 from jaimefrio/bool-ops

ENH: speed-up minimum, maximum and xor for bool dtype