add container v_multiset

tests/container.cpp

@@ -0,0 +1,10 @@
+#include <gtest/gtest.h>
+
+#include "container/container.hpp"
+
+TEST(v_multiset, check)
+{
+  std::vector<int> v;
+  hack::container::v_multiset(v, 1, 2, 3);
+  ASSERT_EQ(v.at(0), 1);
+}