math.powi returns i64, may need cast for int contexts

`math.powi(a, b)` returns `i64`, but Python integer variables map to V's `int`. This usually works fine since V handles int/i64 interop, but could cause type errors in explicitly typed code.

```python
x: int = 2 ** 10
```

```v
x := math.powi(2, 10)  // x is i64, not int
```

Could wrap with `int()` when the target is known to be `int`, but probably not worth the noise for most code.