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/EnumHelper.cpp

@@ -31,7 +31,7 @@ int CheckEnum( lua_State *L, LuaReference &table, int iPos, int iInvalid, const
 		// Get the string and lowercase it
 		lua_pushvalue( L, iPos );
 		LuaHelpers::Pop( L, sLower );
-		sLower.MakeLower();
+		MakeLower(sLower);
 
 		// Try again to read the value
 		table.PushSelf( L );