Use secp256k1_ec_ prefix for non-ECDSA key operations
See API change @ https://github.com/bitcoin/secp256k1/commit/ae6bc76e32016063a591399a7a2d7bac646603bc
Merge pull request #32 from ph4m/master-0.8

Use secp256k1_ec_ prefix for non-ECDSA key operations