[SECP256K1] Make field/scalar code use the new modinv modules for inverses
Summary:
Partial backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/1e0e885c8ac814c3621d9e43e66d60f25e324e8e
Depends on D9404.
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9405