replace outdated rust-crypto crate with RustCrypto/hashes

[AnderEnder]

Issue: rust-crypto is unmaintained and outdated crate:

• https://www.reddit.com/r/rust/comments/dfm3ya/there_are_231_crates_which_might_depend_on_this/
• https://rustsec.org/advisories/RUSTSEC-2016-0005.html

Implementation: replace rust-crypto with RustCrypto crates

[alexcrichton]

Hm sorry I don't really remember much of this code, so if I was the one that added rust-crypto at some point I don't really remember that either. I'm probably not the best reviewer for this :(

[luser]

There's an existing PR for this work that's been sitting due to some open questions: #310

[chmanchester]

Ah, ok, I figured this would be a trivial issue for someone familiar with the rust crypto ecosystem, but I will take a deeper dive into the context in #310 and figure out what to do.

[chmanchester]

I ended up taking care of this in #570, thank you for bringing this to our attention!

[AnderEnder]

@chmanchester, why the PR was not reviewed and closed, but all changes were moved to another one?

[chmanchester]

@AnderEnder by the time I got around to testing the change I found an issue, made a fix locally, and also noticed things needed to be re-based, so it seemed easier to start over. Sorry about that, we should have gotten to this earlier.