Remove orphan instance from `Botan.RNG`

The `Botan.RNG` module from the `botan` package contains an orphan instance of the form:

```hs
instance (MonadIO m) => StatefulGen RNG m where
```

Ideally we would replace this orphan instance by some non-orphan instances that we need, or we make it so that we can access such instances without orphans, using for example `deriving via` or a custom class.