tweak StringOutputStream use

auto str = StringOutputStream()(bla) use form was not doing copy elision or move, but copy

libs/stream/stringstream.h

@@ -153,3 +153,10 @@ template<typename T>
 inline StringOutputStream& operator<<( StringOutputStream& ostream, const T& t ){
 	return ostream_write( ostream, t );
 }
+
+template<std::size_t capacity = 256, typename ... Args>
+StringOutputStream StringStream( Args&& ... args ){
+	StringOutputStream str( capacity );
+	( str << ... << std::forward<Args>( args ) );
+	return str;
+}