Remove ring dependency

[xfbs]

This PR removes the ring crate, and replaces it with the sha2 crate from the awesome RustCrypto ecosystem.

Pros:

• The ring crate is quite heavyweight, it is basically a replacement for the likes of OpenSSL and BoringSSL. If we only need to hash data with a specific algorithm (this is what we use it for), then it should make a lot more sense to import a crate that only does that algorithm, rather than importing the swiss army knife but only using the toothpick.
• The ring crate uses C (ported from BoringSSL) to implement a lot of things. Not really terrible, it is well-tested. I have run into an issue before where it would not build on RISC-V.
• I love using things that are written in pure Rust, which sha2 is.

Cons:

• We have a dependency on rustls, which uses the ring crate anyways.
• Normally, throwing out ring should mean a reduction in binary size, but due to our dependency on rustls this is not the case. I have tested it using cargo bloat and there is no meaningful difference.

Feel free to close this PR if we don't agree that it makes sense!

Cheers!