frequency_map: new merge() method

The stlsoft::frequency_map container class template now includes a merge() method, which allows the contents from two frequency map instances to be merged into one, as in:


  typedef std::string string_t;
  typedef stlsoft::frequency_map<string_t> fmap_t;

  fmap_t map1;
  fmap_t map2;

  map1.push("key-11");
  map1.push("key-22");

  XTESTS_TEST_BOOLEAN_FALSE(map1.empty());
  XTESTS_TEST_INTEGER_EQUAL(2u, map1.size());
  XTESTS_TEST_INTEGER_EQUAL(1u, map1["key-11"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map1["key-22"]);

  map2.push("key-11");
  map2.push("key-21");

  XTESTS_TEST_BOOLEAN_FALSE(map2.empty());
  XTESTS_TEST_INTEGER_EQUAL(2u, map2.size());
  XTESTS_TEST_INTEGER_EQUAL(1u, map2["key-11"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map2["key-21"]);

  map1.merge(map2);

  XTESTS_TEST_BOOLEAN_FALSE(map1.empty());
  XTESTS_TEST_INTEGER_EQUAL(3u, map1.size());
  XTESTS_TEST_INTEGER_EQUAL(2u, map1["key-11"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map1["key-21"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map1["key-22"]);

  XTESTS_TEST_BOOLEAN_FALSE(map2.empty());
  XTESTS_TEST_INTEGER_EQUAL(2u, map2.size());
  XTESTS_TEST_INTEGER_EQUAL(1u, map2["key-11"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map2["key-21"]);

  map2.clear();

  XTESTS_TEST_BOOLEAN_FALSE(map1.empty());
  XTESTS_TEST_INTEGER_EQUAL(3u, map1.size());
  XTESTS_TEST_INTEGER_EQUAL(2u, map1["key-11"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map1["key-21"]);
  XTESTS_TEST_INTEGER_EQUAL(1u, map1["key-22"]);

  XTESTS_TEST_INTEGER_EQUAL(0u, map2.size());
  XTESTS_TEST_BOOLEAN_TRUE(map2.empty());