orion for cryptographic functionality

[brycx]

This PR introduces several changes:

• As previously discussed, this replaces the ring and chacha20-poly1305-aead dependencies with orion to provide XChaCha20Poly1305 and thereby also removes the internal HChaCha20 implementation.
• Manual PartialEq implementation for the Branca type so that comparison on the secret key runs in constant-time.
• Manual implementation of Debug to avoid the secret key leaking into logs.
• Checking expired tokens (timestamp + ttl) is now done with checked_add to avoid a silent wrap-around on release builds.

[charleschege]

is it okay to use orion given that on its page author recommends not to use for production ?

[brycx]

is it okay to use orion given that on its page author recommends not to use for production ?

That's a very valid concern. I am the author of orion. The warning about production use comes mainly from the fact, that there has not been a third-party security audit done on any current version of the library. It's also relatively new and has not seen that much use yet. If you're interested in the security and testing of it, I can recommend looking at the wiki.

I have been in touch with @return about using orion and we settled upon me opening this PR.

[return]

@charleschege As with several other Rust cryptography crates, I will have to put a warning notice up for using branca in a production environment since orion doesn't have a formal security audit yet. @brycx aims for orion to have a security audit soon.

But in comparison to the previous crate that was used, orion is actively maintained and supports the full XChaCha20-Poly1305 construction in pure rust which was a hard requirement that was needed in branca. Which was the reason why I chose to use orion.

@brycx Thanks for your pull request and I'll squash your commits and merge this in now.