public static Scalar128 <T> ToScalar128 <T>(this ReadOnlySpan128 <T> src, int block = 0)
where T : struct
=> Num128.load(src, block);
public static Scalar128 <T> ToScalar128 <T>(this Vec128 <T> src, int index)
where T : struct
=> Num128.define(src[index]);