Relax the `Sized` requirement on the random source generic

[sandhose]

This removes the Sized requirement on the random source generic. I need this because &dyn Traits are not !Sized, so the following code would not work before, but it does now:

fn f(rng: &mut dyn RngCore, now: SystemTime) -> Ulid {
    Ulid::from_datetime_with_source(now, rng)
}

fn main() {
    f(&mut rand::thread_rng(), SystemTime::now());
}

[sandhose]

@dylanhart Thanks for the merge! Would you mind releasing a new version with it included? I currently have to override the ulid dependency in my project – which is fine –, but I'd like to get back to a version released on crates.io rather sooner than later