BigInt arithmetic `op_add` is buggy

```moonbit
-1N + 1N == 0 // false
println(-1N + 1N) // panics
```

In fact, `-1N + 1N` produces the following result:
```
{
 limbs: [],
 sign: Positive,
 len: 0
}
```

but we expected `0N`:
```
{
 limbs: [0],
 sign: Positive,
 len: 1
}
```