Abstract out challenge computation in schnorrsig
Abstract out challenge computation in schnorrsig

peterdettman/secp256k1,peterdettman/secp256k1,evoskuil/secp256k1,afk11/secp256k1,evoskuil/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin-core/secp256k1,afk11/secp256k1,evoskuil/secp256k1,afk11/secp256k1,evoskuil/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,afk11/secp256k1