Method on_comment_part

test/basic_parser.cpp:983–987

            bool on_bool( bool, system::error_code& ) { return true; }
            bool on_null( system::error_code& ) { return true; }
            bool on_comment_part( string_view s, system::error_code& )
            {
                captured.append(s.data(), s.size());
                return true;
            }
            bool on_comment( string_view s, system::error_code& )
            {
                captured.append(s.data(), s.size());