Implement ChaCha20 about ocaml-salsa20-core

[abeaumont]

Hi @cfcs!
Note that this repo is rather minimalistic and it only implements Salsa20 Core functions, not even the Salsa20 encryption function (which I've mostly working at https://github.com/abeaumont/ocaml-salsa20). The reason for this is that Salsa20 Core functions are also used outside of Salsa20 encryption function (i.e. https://github.com/abeaumont/ocaml-scrypt-kdf).

Following this reasoning, I think that ChaCha20 deserves its own repo/package. Even if it's only slightly different to Salsa20, its differences are at all levels (quarterrounds, rounds, matrix, etc.), so at the end there's not much code to reuse (and the code is quite simple anyway).

That being said, I also want to see ChaCha20 implemented, so I may just create the new repo and write it. If you're in a rush or want to do it yourself please go ahead and just let me know, and I'll try to contribute as well.

[cfcs]

PS: If you don't have time and would rather accept a PR, I'd be happy to do that. :)

[cfcs]

Alright, that makes sense :) I have only done very little OCaml<->C integration, so I think you're much better equipped to do it than I am. I'll need it in 2-3 months from now, so I'll give it a go by then if this pesky "real life" thing has held you up; otherwise I'll just opam install chacha20 and live happily ever after. I'll ping you if I start writing code :)

[abeaumont]

Ok, I've created the repo https://github.com/abeaumont/ocaml-chacha20, you can watch it to keep updated =)
Closing this.