Add 'ostream' method to class StreamBuffer, and use it.

luprex/core/cpp/drivenengine.hpp

@@ -246,6 +246,10 @@ public:
     //
     Channel *get_stdio_channel();
 
+    // Obtain the output buffer of the stdio channel as an ostream.
+    //
+    std::ostream &stdostream() { return get_stdio_channel()->out()->ostream(); }
+
     // Fetches the lua source, and takes ownership of it.  The DrivenEngine
     // no longer contains the source after calling this.
     //