Cuddle can't generate (or parse rules) that involve literal bigints

With this cddl file:

```
a_fixed_bigint = -69820359824690251347463789091535876986214416475039738375086693072001776
```

`cuddle gen` produces:
```
➜ doRule a_fixed_bigint
3b 273920e1e0305aef # negative(2826326395652692719)
```

it parses the rule as: `Literal (VNInt 2826326395652692720)`.

whereas cddl does generate this value:
```
➜ cddl b.cddl generate
-69820359824690251347463789091535876986214416475039738375086693072001776
```