Make the curve B constant a secp256k1_fe
Make the curve B constant a secp256k1_fe

Summary:
This is a partial backport of secp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/808 | 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

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