Bitcoin Core changed the C API for BIP340 Schnorr signatures within secp256k1. We don't use them in Bitcoin ABC, so thus far this backport was not needed.
This is a backport of [[ https://github.com/bitcoin-core/secp256k1/pull/850 | secp256k1#850 ]].
Now we do need this backport mostly for `secp256k1_schnorrsig_sign_custom` for porting `rust-secp256k1` to the secp256k1 library of this repo.
Depends on D16957.