title: Concat / Join / Transform multiple columns to one struct column
body: <p>I have very big, legacy file with ~5000 columns and very big amount of record.
  Many columns are named like <code>a_1,a_2,...,a_200</code> etc.</p>
  <p>I want to concatenate number of columns into struct (for better data manipulation later), so instead:</p>
  <pre><code>_| a_1 | a_2 | a_3 |...
  0| true | false | true |...
  1| false | true | false |...
  </code></pre>
  <p>I would like to have struct <code>a { 1: true, 2: false, ... 200: true }</code> .</p>
  <p>How to transform it using Python, probably Panda's?
  Columns have always same prefix, like a_, b_... etc.</p>
  <p>Greeetings</p>