This is in preparation for allowing code reuse by xonly tweak add functions
This is a partial backport of secp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/558 | PR558]] : https://github.com/bitcoin-core/secp256k1/pull/558/commits/176bfb1110147b5dca1834ea071acc846fb1cab3