refactor: re-place files into correct position according to namespace hierarchy

testbench/yycc/rust/num/stringify.cpp

@@ -0,0 +1,14 @@
+#include <gtest/gtest.h>
+#include <yycc.hpp>
+#include <yycc/rust/num/stringify.hpp>
+
+#include <yycc/prelude/rust.hpp>
+
+#define STRINGIFY ::yycc::string::stringify
+
+namespace yycctest::rust::stringify {
+
+    // There is not testbench for this
+    // because it just a map to original implementation without any modification.
+
+}