[DO NOT MERGE] Schnorr batch verification for blocks

This is a simple draft implementation of Schnorr signature batch verification in blocks, put here for conceptual discussion, and collaborative benchmarking. Most of code added here is from the secp256k1 implementation.

The secp256k1 code is still under review itself, please spend some time giving feedback on the API etc. if you can:

• Original PR
• My rebase used here including some minor fixes

TODOs

• Fix functional/feature_taproot.py
• Extensive benchmarking using ConnectBlock() tracing
• Add unit tests
• Batch taproot tweak verification as well
• Conceptual discussion in what form this realistically could be introduced into the code base, and later, a release