Int: Support big.Int

Currently `Int(...)` only supports `int64` which has a [limit](https://pkg.go.dev/math#pkg-constants).

Implement utility function to support [`big.Int`](https://pkg.go.dev/math/big#Int) to be able to go much higher, possibly with a new soft limit at a [googol](https://en.wikipedia.org/wiki/Googol). Anything higher would just result in `x googols`.