is_keyword(): pass argv by const reference

src/parser.cc

@@ -131,7 +131,7 @@ public:
   }
 
   bool
-  is_keyword(std::string p_argument)
+  is_keyword(const std::string &p_argument)
   {
     std::set<std::string> keywords = { "[", "]", ":" };
     auto funcIter = keywords.find(p_argument);