Add k12

[dhardy]

I wrote a KangarooTwelve implementation. It's slow and has horrible code style, but passes the tests at least. Licenced CC0 so feel free to copy.

https://github.com/dhardy/hash-bench/blob/master/src/k12.rs

[quininer]

I have some interest in MarsupilamiFourteen.

[DonaldTsang]

@quininer nice to see people care about K12 in here

[oconnor663]

If anyone working on this wants to benchmark against the official K12 implementation, I have some very simple Rust bindings that you could copy, which statically link that implementation. The vendored version of that implementation (just updated) does AVX2 and AVX-512 feature detection at runtime.