Make the curve B constant a secp256k1_fe

Summary:
This is a partial backport of secp256k1 PR808 : https://github.com/bitcoin-core/secp256k1/pull/808/commits/78f6cdfaae9866694dcb0eee966332688753a8c3

Depends on D7657

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7658