Change some RString methods to free functions

These ones aren't a std::string method. Doing this helps the RString to
std::string migration.

src/Difficulty.cpp

@@ -79,7 +79,7 @@ OldStyleStringToDifficultyMapHolder OldStyleStringToDifficulty_converter;
 Difficulty OldStyleStringToDifficulty( const RString& sDC )
 {
 	RString s2 = sDC;
-	s2.MakeLower();
+	MakeLower(s2);
 	std::map<RString, Difficulty>::iterator diff=
 		OldStyleStringToDifficulty_converter.conversion_map.find(s2);
 	if(diff != OldStyleStringToDifficulty_converter.conversion_map.end())