Go: refactor (#16602)

### Summary

1. update doc
2. refactor route code

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/binding/cpp/util/util.h

@@ -0,0 +1,44 @@
+// Copyright 2009 The RE2 Authors.  All Rights Reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifndef UTIL_UTIL_H_
+#define UTIL_UTIL_H_
+
+#define arraysize(array) (sizeof(array) / sizeof((array)[0]))
+
+#ifndef ATTRIBUTE_NORETURN
+#if defined(__GNUC__)
+#define ATTRIBUTE_NORETURN __attribute__((noreturn))
+#elif defined(_MSC_VER)
+#define ATTRIBUTE_NORETURN __declspec(noreturn)
+#else
+#define ATTRIBUTE_NORETURN
+#endif
+#endif
+
+#ifndef ATTRIBUTE_UNUSED
+#if defined(__GNUC__)
+#define ATTRIBUTE_UNUSED __attribute__((unused))
+#else
+#define ATTRIBUTE_UNUSED
+#endif
+#endif
+
+#ifndef FALLTHROUGH_INTENDED
+#if defined(__clang__)
+#define FALLTHROUGH_INTENDED [[clang::fallthrough]]
+#elif defined(__GNUC__) && __GNUC__ >= 7
+#define FALLTHROUGH_INTENDED [[gnu::fallthrough]]
+#else
+#define FALLTHROUGH_INTENDED                                                                                                                         \
+    do {                                                                                                                                             \
+    } while (0)
+#endif
+#endif
+
+#ifndef NO_THREAD_SAFETY_ANALYSIS
+#define NO_THREAD_SAFETY_ANALYSIS
+#endif
+
+#endif // UTIL_UTIL_H_