`chacha20` - Clone is not implemented for `ChaChaXRng`

[nstilt1]

DHardy mentioned some code that was related to the test_chacha_clone_streams() test, and then I realized that the test doesn't actually perform the clone operation because Clone is not implemented for the RNG. To do this, we would have to implement Clone for ChaChaCore<R, V> or implement Clone for ChaChaXCore manually and clone the ChaChaCore struct from there. I have some questions:

1. Does implementing Clone for ChaChaCore<R, V> pose any security risks for either the RNG or the cipher? Like if it enabled users to clone the cipher/RNG in a way that is less secure?
2. Would we prefer to add impl PartialEq and impl Eq to rng.rs or lib.rs, ie impl them for ChaChaCore<R, V> in lib.rs or only impl them for ChaChaXCore in `rng.rs?

[tarcieri]

We deliberately removed Clone in #462.

Cloning cipher instances can lead to the keystream being duplicated or duplicated RNG outputs, either of which pose severe security risks. Duplicated keystreams cancel each other out leaving the XOR of the plaintexts, and duplicated RNG outputs could expose sensitive secrets e.g. generated cryptographic keys.

[nstilt1]

Ah, okay then. What about the clone test? I suppose we ought to either deprecate it or remove it or rename it. Maybe twin_rng_test? If we remove it, I think it would be nice to leave a comment // fn test_chacha_clone_streams removed in #462, but of course it could be found in the git history.

[tarcieri]

An alternative to cloning is forking the RNG, which will produce distinct outputs from the two RNGs