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

[jasagredo]

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