len</code> copies <code class="code">len</code> weak pointers
   from <code class="code">ar1</code> (starting at <code class="code">off1</code>) to <code class="code">ar2</code> (starting at <code class="code">off2</code>).
   It works correctly even if <code class="code">ar1</code> and <code class="code">ar2</code> are the same.
   Raise <code class="code"><span class="constructor">Invalid_argument</span> <span class="string">"Weak.blit"</span></code> if <code class="code">off1</code> and <code class="code