[SECP256K1] WNAF of lambda_split output has max size 129
Summary:
Partial backport 9/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/2edc514c90293af8f602e4376e832773779c9426
Depends on D8044.
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8045