Add trait ReproducibleRng

[dhardy]

Summary

pub trait ReproducibleRng: SeedableRng {}

Motivation

Clarify that SeedableRng does not imply reproducibility and what RNGs do have this property.

Details

I'm somewhat loathe to do this since in part, reproducibility is a special property that StdRng and SmallRng opt out of. On the other hand, this is a property not implied by the API or the semver model.

So does this form of formal documentation make sense?

Obviously this PR is incomplete: the ChaCha and Pcg RNGs in this repo should implement the trait, and also StepRng and most of the rngs repo.

[newpavlov]

Personally, I think we do not need this trait and that it's sufficient to explain the reproducibility properties in docs.

[dhardy]

Personally I don't either. I'm also not wildly keen on adding it.

It does help clarify what SeedableRng does not offer, but simply adding docs there may be enough.

[benjamin-lieser]

My first thought was also that I do not like this. But on second thought the cost is very low, because it increases the API only by a very small amount and it has a good chance that everyone who cares about reproducibility will do the right thing without reading the docs too closely.

So I am in favor, but no strong opinion