mirage-crypto-ec: implementation of SECP256K1 This change implements the SECP256K1 curve (also known as the Bitcoin curve). - field primitives are generated by the fiat-crypto project[1] - point primitives are generated by the ECCKiila project[2] - Ocaml point operations are taken from NIST implementation, adapted to ECCKiila point primitives and optimized for a=0. - testvectors for ECDH and ECDSA verification from wycheproof[3] Closes: #187 [1] https://github.com/mit-plv/fiat-crypto [2] https://gitlab.com/nisec/ecckiila [3] https://github.com/C2SP/wycheproof