Kupyna hasher

[jkoudys]

We've implemented this hashing algorithm for kupyna, a Ukrainian algo specified here: https://eprint.iacr.org/2015/885.pdf

I'm opening this PR just to get the ball rolling. The hash function we've made works and we've included a bunch of test units. Next step is to implement the Digest and associated RustCrypto traits. I don't think this should go into RustCrypto/hashes master branch, but figured I'd open this and you could setup a new branch (or let us know the best way to build this). Any tips you could give to get us started on impl'ing the Digest trait would be appreciated.

Is there an existing hash implementation with a variable size hash? Most seem to hardcode it, but kupyna allows from 8 to 512, so it ought to be part of its digest struct. Is there a precedent for this already? I'd like to avoid reinventing the wheel.