Adding secp256k1 support

[infinity0]

lightning uses noise-with-secp256k1 even though this instantiation is not explicitly mentioned in the noise protocol spec.

Are there any plans to add this to cacophony, or would you be happy to accept a PR implementing such support?

[centromere]

I would accept a PR providing such support.

[ovangle]

Are you OK with secp256k1 from cryptonite lib? If so I could create a PR for that.

[centromere]

Yes. I am OK with that because it doesn't introduce an additional dependency.

[ysangkok]

I have been working on this. We need to support the custom key rotation mechanism they use in Lightning, but if we do, it will work. I have a PR for secp256k1 support in Cryptonite, let's see if I can get that merged. If I can't, would you be ok with using the ECDH bindings that I could add to secp256k1-haskell? That would require one new dependency then.

[centromere]

Yes.

[ysangkok]

See PR #13