title: Compute hash of map structure
body: <p>I would like to compute a hash value of an <code>unordered_map</code> data structure as a whole. This enables to easily compare two <code>map</code>s whether they contain exactly the same key-value-pairs or not.</p>
  
  <p>Obviously, one can iterate over the included pairs, build a long string an hash it afterwards but I can imagine that there are better ways to do this.</p>
  
  <p>For the moment, the actual hash function is not that crucial. I think <code>md5</code> would be ok. <code>sha</code> too of course. </p>
  
  <p>Are there any suggestions?</p>