Shifts right using at most size_t.sizeof * 8 - 1 bits
auto a = UInt!128.fromHexString("afbbfae3cd0aff2714a1de7022b0029d"); assert(a.smallRightShift(4) == UInt!128.fromHexString("afbbfae3cd0aff2714a1de7022b0029"));
See Implementation
Shifts right using at most size_t.sizeof * 8 - 1 bits