.log to print success value

For those, who have to support existing parsers written with fastparse, it would be a very nice idea to add `fastparse.ParsingRun#successValue` to `fastparse.LogByNameOps#log` output, e.g. 

```
-primary:1:1:Success(1:2, cut) => Integer(1)
```

Printing only the first 64 characters of string representation would make the most sense.