Constant string matches are not reported

```
grammar G {
        token TOP { <a> b <c> }
        token a { a }
        token c { c }
}

say so G.parse('abc');
```

```
TOP
|  a
|  * MATCH "a"
|  c
|  * MATCH "c"
* MATCH "abc"
True
```

There is no mention of 'b' being matched until whole TOP matched. In complex grammars it's difficult to spot what last thing was matched before failure.
