Add functions to simplify creating sub-RNGs

[hjooh]

Addresses #3774. I updated the Rand trait to have a method (create_sub_rng) that creates a sub-RNG seeded from the current RNG state and added a simple method (sub_rand) to the HasRand trait for the sake of convenience when calling this function.

I also made a small update to MIGRATION.md (after reading through CONTRIBUTION.md) and added a function to test my changes. As this is my first attempt at contributing to this repository, I would really appreciate any and all feedback. Thank you!

[addisoncrump]

Sorry to nitpick, but there are some design issues to address here.

[hjooh]

Just to follow up: I've made the requested changes, but had some thoughts about the best way to go about it. My code is as follows:

/// Produce a sub-RNG seeded from the current RNG state.
/// Useful when sampling random data while also accessing the state.
pub trait SubRng {
    /// Creates and returns a sub-RNG.
    fn sub_rng(&mut self) -> Self;
}

impl<R: Rand + Sized + Clone> SubRng for R {
    /// Creates and returns a sub-RNG.
    fn sub_rng(&mut self) -> Self {
        let mut sub = self.clone();
        sub.set_seed(self.next());
        sub
    }
}

SubRng no longer has Rand + Sized + Clone, and I've removed the default implementation from the trait body and moved the logic to the impl block. I noticed that there were multiple RNGs (StdRand, Sfc64Rand, etc.) so I assumed that this would be the best approach, but I'm happy to change things if needed. Thank you again for the feedback!

[addisoncrump]

I made some final changes to clean up some lints + use preferred formatting.