Digest and Cipher don't implement Zeroize

[kayabaNerve]

We can place them in a Box using a different allocator which does zero deallocated memory however.

https://github.com/kayabaNerve/healthy-paranoia/blob/develop/allocator/src/lib.rs for reference.

[kayabaNerve]

This requires nightly, yet is available on the zeroizing-allocator branch. I did not update Ciphersuite::hash_to_F, solely the DKG's Chacha20, and then the message-box branch also has an untouched Chacha20.

Until this moves off of nightly, no further progress can be made.

[kayabaNerve]

We can merge zalloc as is. It's perfectly fine and has a stable and nightly offering. That would enable using it in the new MessageBox key_gen binary.

[kayabaNerve]

#154 was merged so it can be used in the aforemention key_gen binary. It's cherry-picked from https://github.com/serai-dex/serai/tree/zeroizing-allocator as to not edit dkg in ways which would require nightly.

35ca220 fixed the CI which was activating the nightly only feature. While there's probably a better method available, this is fine.

[kayabaNerve]

The zeroizing-allocator branch was only ahead by c9d89c9. That is stale as of 13977f6. I've deleted the branch accordingly, and that commit has TODO notices for when Box allocators are no longer nightly only.

[kayabaNerve]

Implementors of Digest do generally implement ZeroizeOnDrop now, so this can likely be closed after RustCrypto/traits#2353 🎉