I adapted `p256-m` for SEC-P256K1 (Koblitz)

[cobratbq]

Hi, I adapted p256-m for the Koblitz-curve p256k1 for my use-case. FYI. In case you're interested.

Fwiw, I haven't done constant-time verification and secret-value-poisoning yet, because I focused on functionally operable and correct first, i.e. priorities.

• p256-m for SEC p256k1 (Koblitz-curve)
• extensions for Schnorr-signing

I am, most likely, less experienced with such an implementation, so I worked off your foundation and used the formulas mentioned in the comments.