Make ChaCha20Rng zeroizable

[kustosz]

ChaCha20Rng does not currently implement Zeroize. It seems like there is no trivial way to implement it, so it either requires an upstream contribution or unsafe memory fiddling. Either way, this behavior should be implemented and used in batch_contribution::derive_taus.

[kustosz]

Judging by the discussions in rust-random/rand#934, an upstream contribution could be considered.

[nstilt1]

I've submitted a pull request for rand_chacha's dependency, and I have some code available for rand_chacha once the ppv_lite86 PR gets approved, but it requires a Cargo.toml change that includes the next version of ppv_lite86 before rand_chacha can be updated. After rand_chacha is updated, this repo should be able to be updated