Int128.opCast

const Int128 a = Int128(0xffff_ffff_ffff_ffffL, 0x0123_4567_89ab_cdefL);
assert(cast(long) a == 0x0123_4567_89ab_cdefL);
assert(cast(int)  a ==           0x89ab_cdef);
assert(cast(byte) a == cast(byte) 0xef);