Remove converter hack around std::map::iterator

[lucic71]

The old code special cased std::map::iterator (which is just a sugar for std::pair<const K, V>) to generate i.first() and i.second() instead of i.0 and i.1.

This PR removes the hack by saving a pair in libcc2rs::MapIter which can be accessed with .0 and .1. It also fixes a translation bug:

 for (auto i : m) {
    i.second = -1;
    assert(i.second == -1);
  }
  for (const auto &i : m) {
    assert(i.second != -1);
  }