title: How to merge dicts, collecting values from matching keys?
body: <p>I have multiple dicts (or sequences of key-value pairs) like this:</p>
  <pre><code>d1 = {key1: x1, key2: y1}
  d2 = {key1: x2, key2: y2}
  </code></pre>
  <p>How can I efficiently get a result like this, as a new dict?</p>
  <pre><code>d = {key1: (x1, x2), key2: (y1, y2)}
  </code></pre>