title: Group by key + merge array values
body: <p>I'm trying to merge the array values of multiple arrays with same keys (sort of group by + merge).  Is there any native way in Go to transform this input:  </p>
  
  <pre><code>input = [
      [
          { a: 1, b: [1,2]},
          { a: 2, b: [1,2]},
          { a: 3, b: [1,2]}
      ],
      [
          { a: 1, b: [3,4]},
          { a: 2, b: [3,4]},
          { a: 3, b: [3,4]},
          { a: 4, b: [3,4]}
      ],
      [
          { a:1, b: [5,6]},
          { a:2, b: [5,6]},
          { a:3, b: [5,6]},
          { a:4, b: [5,6]},
          { a:5, b: [5,6]}
      ]
   ]
  </code></pre>
  
  <p>into:</p>
  
  <pre><code>output = [
      { a: 1, b: [1,2,3,4,5,6]},
      { a: 2, b: [1,2,3,4,5,6]},
      { a: 3, b: [1,2,3,4,5,6]},
      { a: 4, b: [3,4, 5,6]},
      { a: 5, b: [5,6]},
  ]
  </code></pre>