Output of listing/0 doesn't quoted atoms

Example below. Note how neither atom is quoted and raw newlines are output.

I think the output of listings should be quoted, so it can be re-read.

```
?- assert(fact('file.txt', 'format: $s\n$s\n')), listing.

% user:fact/2
fact(file.txt,format: $s
$s
).
```