title: JQ: add nested object with name of top-level key
body: <p>In my input in JQ 1.4...</p>
  
  <pre><code>{
      "key x": {
          "b": "1"
      },
      "key y": {
          "b": "1"
      }
  }
  </code></pre>
  
  <p>...should add new nested objects under each top-level key with the name of its  top-level key. So the result looks like</p>
  
  <pre><code>{
      "key x": {
          "a": "key x",
          "b": "1"
      },
      "key y": {
          "a": "key y",
          "b": "1"
      }
  }
  </code></pre>
  
  <p>But without using the key names (e.g. ."key x".a="key x") directly, because i dont know during runtime the real key names.</p>