Add keccak crate

[RalfJung]

Quoting @oli-obk:

The keccak crate is a great benchmark for ridiculous numbers of locals and basic blocks. We should add it to the list of perf tests.

So, let's do that. :) This adds an old version, one that still actually has "ridiculous numbers of locals and basic blocks". Later versions reduced that, also to help with compile-times -- but the old version is a good stress-test, so I think it is the better benchmark.
Concretely, this is an exact copy of https://github.com/RustCrypto/sponges/tree/a417e97a41776a2feee20f98e1d97917b7ad35f0/keccak.

Is there anything else needed, other than adding it in this directory?

[RalfJung]

I wanted to also add a benchmark @oli-obk and me developed, but as rust-lang/rust#52849 (comment) shows it is currently really slow. So I guess I better wait?

[Mark-Simulacrum]

Pending CI passing this looks good.

I think I'd be fine to add the other benchmark if it's actually 30 seconds -- many of our benchmarks are ~5-10 minutes long so a 30 second or minute long build won't do all that much :)

[RalfJung]

It's 30 seconds after reducing complexity.^^ The original one (which takes <1min on a nightly from a week ago) did not complete compilation in 7min, then I stopped it...

[Mark-Simulacrum]

Ah, okay, then yeah, let's hold off on that. Looks like CI here has passed so merging and deploying shortly.

[RalfJung]

Great, thanks!

FWIW, keccak regressed 2% since 2018-07-07. Doesn't seem big enough to track separately, I guess. Also maybe fixing rust-lang/rust#52849 will fix that as well.