MAINT: silence warning by casting to right type
MAINT: add npy_memchr for memchr like operations

useful for small sizes where the function call is expensive.
ENH: improve npy_memchr(p, 0) by using __builtin_ctz (tzcnt on x86)

Improves sparse mask performance by about a factor of three, the worst
case of no consecutive mask elements slows down by about 10%-15%.