title: How do I convert a string to a lower case representation?
body: <p>How do I convert a string to a lower case representation?</p>
  
  <p>I feel that there must be built-in function for it, but I just can't find it.</p>
  
  <p>I did find a <code>ToLower</code> in <code>"unicode/letter"</code>, but it only works for one rune at a time.</p>