[SECP256K1] Reorder comments/function around scalar_split_lambda

Summary:
Partial backport 10/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/63c6b71616816b19bec9cb3ab6b45ae5afd955f0

Depends on D8045.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8046