Function croaring_sys::fast_union_uint16

pub unsafe extern "C" fn fast_union_uint16(
    set_1: *const u16, 
    size_1: size_t, 
    set_2: *const u16, 
    size_2: size_t, 
    buffer: *mut u16
) -> size_t

combines union_uint16 and union_vector16 optimally