Add fmt::to_string (#326)

test/string-test.cc

@@ -0,0 +1,15 @@
+/*
+ Tests of string utilities
+
+ Copyright (c) 2012 - 2016, Victor Zverovich
+ All rights reserved.
+
+ For the license information refer to format.h.
+ */
+
+#include "fmt/string.h"
+#include "gtest/gtest.h"
+
+TEST(StringTest, ToString) {
+  EXPECT_EQ("42", fmt::to_string(42));
+}