Add ECDSA for the secp256k1 curve

[briansmith]

We can probably add support for the secp256k1 curve pretty easily mostly by copy/pasting the P-384 code and changing the constants (number of limbs: 4, new q and n values). It probably won't be as fast as libsecp256k1 but it would be good enough for most uses.

[Zeromika]

I suppose this is still available, mind if I take a look @briansmith?

[except]

Any chance this is in the works? I was looking to use implementation over others.

[briansmith]

Do you need signing or just verification?

[briansmith]

Closing this since I think people who want secp256k1 support already are satisfied with other solutions