Optimization: use formulas instead of lookup tables for cancelling g bits
Optimization: use formulas instead of lookup tables for cancelling g bits

This only seems to be a win on 64-bit platforms, so only do it there.

Refactored by: Pieter Wuille <pieter@wuille.net>

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