World serialization done (not well-tested yet)

luprex/core/cpp/streambuffer.hpp

@@ -302,6 +302,7 @@ public:
     int64_t write_count() const;
 
     // Delete all data and (if not fixed-size) free the buffer.
+    // Also resets the read and write counts.
     void clear();
 
     // Alloc_space and wrote_space need to be invoked together.