Add SmallThreadRng

[adityashah1212]

Background

What is your motivation?
Same use case as ThreadRng serves today, but something that is faster than ThreadRng while sacrificing crystallographic security.

What type of application is this? (E.g. cryptography, game, numerical simulation)
Simple stuff like a randomized log sampler. Using rand::small_random() is much easier than

thread_local! {
    static RNG: RefCell<SmallRng> = RefCell::new(SmallRng::from_rng(&mut rand::rng()));
}

fn should_emit() -> {
   RNG.with_borrow_mut(|rng| rng.random_ratio(1, 1000)
}

Feature request

Create a type called SmallThreadRng that internally uses same rng as SmallRng.