`rand::RngCore` implementation for `numpy.random.Generator`

We should allow a way to pass a `numpy.random.Generator` to a pyo3 function and use it on the Rust side by something requiring a `rand::Rng` by implementing [`rand::RngCore`](https://docs.rs/rand/latest/rand/trait.RngCore.html).

See
- https://numpy.org/doc/stable/reference/random/c-api.html
- https://numpy.org/doc/stable/reference/random/extending.html