Add suport for additional hash algorithms

[heartsucker]

Currently the plan is to only use sha256 and sha512.

Note: sha1 and md5 are out of the questions. If someone wants to tackle this, don't implement those.

[trishankkarthik]

Nice! We're planning to support SHA-3 soon enough...

[heartsucker]

Yeah, I meant I'm only going to support those two initially because I have to start with something. Ideally there'd be a large number of options in the lib so people can pick and choose what works.

[tonychain]

SHA-256 and SHA-512 are fine, FWIW.

[heartsucker]

I mean, yeah, those two are sufficient for most use cases, but since this is a lib, I want it to be flexible for other people's use cases. Also, this is probably something relatively straight forward a new contributor could do to get a feel for the codebase.

[tonychain]

I left some notes on the upstream TUF thread. If you want a SHA-3 family function, I'd suggest using SHAKE128 instead of SHA3-256 simply because it's faster (and that's more or less advice directly from the Keccak Team, as it were)

[heartsucker]

@tonychain The spec doesn't specify which hash algorithms to use. here. To that end, if this is going to flexible, we should add in whatever (safe, modern) hash algorithms other people want to use.

[heartsucker]

Blocked by briansmith/ring#59