Fix and extend map test
Fix and extend map test

 * Fix "multimap serialization" test: did not test multimap before
 * Add test of class with json member for both value and key
 * Shift class names around a bit to get comparable to vector_test:
   * old X is now A<M>
   * new X is a wrapper around an int member

d-led/picojson_serializer,d-led/picojson_serializer