Switch from cffi libsecp256k1 to ironclad secp256k1

[kilianmh]

Since this commit sharplispers/ironclad@4c689d0 Ironclad supports libsecp256k1. It would be more convenient to not depend on external libraries with cffi but use cl-native code. If you need some help I can try to assists (even though my cryptographic knowledge is also limited).

[rodentrabies]

Thanks for the suggestion. I'd still want the default to be the CFFI bindings to the C library for security purposes, but having the ironclad implementation as fallback is definitely convenient. I'll work on it at some point.