feat: convert signing to k256

[gakonst]

Supersedes #67 by @roynalnaruto (rebased / cherry picked on master due to lots of changes since the original PR)

[shamatar]

@gakonst Georgios, you should be able to fix the PR and use it now

[gakonst]

Signature verification works now with the new method added here RustCrypto/elliptic-curves#205. We're only missing a method to also sign a prehashed message now, to replace the SigningKey.sign call, and then we should be good

[tarcieri]

It might be a bit more straightforward to use RecoverableSignPrimitive:: try_sign_recoverable_prehashed (which is impl'd on k256::Scalar) for this:

https://docs.rs/ecdsa/0.8.3/ecdsa/hazmat/trait.RecoverableSignPrimitive.html

The API to compute an ephemeral scalar with RFC6979 is public:

https://docs.rs/ecdsa/0.8.3/ecdsa/rfc6979/fn.generate_k.html

You can convert the raw message digest to a scalar using Scalar::from_bytes_reduced

[gakonst]

@tarcieri wasn't sure how to do it exactly, ended up going with @shamatar's solution which does the job! Thank you @shamatar @roynalnaruto @tarcieri for your help, this was great (and @tuxxy for the original k256 impl).