Add: LinkBehaviour to AddTo

[nuskey8]

No description provided.

[akeit0]

This seems good.
However, there is one improvement, it would be better if the initialization of the List is lazy.

struct LazyFastListCore<T>{
    public static int InitialCapacity=8;
    T[] array;
    int tailIndex;
    void AllocateIfNeeded(){
         if(array==null)array=new T[InitialCapacity];
    }

     public void Add(T element)
     {
         AllocateIfNeeded();
          if (array.Length == tailIndex)
          {
              Array.Resize(ref array, tailIndex * 2);
          }

          array[tailIndex] = element;
          tailIndex++;
      }

     public readonly Span<T> AsSpan() => array==null?default:array.AsSpan(0, tailIndex);
}

[nuskey8]

Yes, I see that. If so, it would be better to delay the FastListCore array generation in the first place. Once this PR is merged, another PR will take care of it immediately.

[nuskey8]

#107