title: Merge nested dictionaries with common / overlapping keys
body: <p>I have two dictionaries, I want to merge following two dictionary into one.</p>
  
  <p>How can I merge them efficiently?</p>
  
  <p><strong>Dict1</strong></p>
  
  <pre><code>{
      UUID('id1'): {
          'due': datetime.date(2018, 3, 10),
          'status': 'Done',
      }, UUID('id2'): {
          'due_date': datetime.date(2018, 3, 10),
          'old_status': 'Done',
      }
  }
  </code></pre>
  
  <p><strong>Dict2</strong></p>
  
  <pre><code>{
      UUID('id1'): {
          'new_val': 'Pass',
          'amount': 123.0,
      }, UUID('id2'): {
          'new_val': 'Fail',
          'amount': 0,
      }
  }
  </code></pre>
  
  <p><strong>Desired Output</strong> </p>
  
  <pre><code>{
      UUID('id1'): {
          'due': datetime.date(2018, 3, 10),
          'status': 'Done',
          'new_val': 'Pass',
          'amount': 123.0,
      }, UUID('id2'): {
          'due_date': datetime.date(2018, 3, 10),
          'old_status': 'Done',
          'new_val': 'Fail',
          'amount': 0,
      }
  }
  </code></pre>