public static Vec128 <ulong> ToVector128(this UInt128 src) => src;
public static unsafe ref UInt128 mul(ulong lhs, ulong rhs, out UInt128 dst) { dst = 0; mul(lhs, rhs, out dst.lo, out dst.hi); return(ref dst); }
public static BitString ToBitString(this UInt128 src) => BitString.FromScalar(src.hi) + BitString.FromScalar(src.lo);