title: Melt a data.table with a list data type (get lists of values in one col for each unique value in another col)
body: <p>I have a <code>data.table</code> that looks like this,</p>
  
  <pre><code>dt &lt;- data.table(
         cbind(
            id = c( 1, 1, 2, 2, 3, 3, 3 ),
            y = c( 'a', 'b', 'c', 'd', 'e', 'f', 'g' )
         )
  );
  </code></pre>
  
  <p>I would like to transform it to a <code>data.table</code> that looks like this:</p>
  
  <pre><code>   id   y.list
  1:  1    list( a, b )
  2:  2    list( c, d )
  3:  3    list( e, f, g )
  </code></pre>
  
  <p>The <code>list</code> above can be any container, i.e. either a <code>vector</code>, a simple <code>c</code>, or any other container that can be iterated upon later on. Grateful for any suggestions!</p>