libgolang: Provide std::hash<refptr>

So that refptr<T> could be used as keys in e.g. dict (unordered_map) or
set (unordered_set). set<refptr> will soon be used by C version of
context package.

golang/libgolang.h

@@ -605,6 +605,20 @@ public:
 
 
 }   // golang::
+
+
+// std::hash<refptr>
+namespace std {
+
+template<typename T> struct hash<golang::refptr<T>> {
+    std::size_t operator()(const golang::refptr<T>& p) const noexcept {
+        return hash<T*>()(p._ptr());
+    }
+};
+
+}   // std::
+
+
 #endif  // __cplusplus
 
 #endif  // _NXD_LIBGOLANG_H