[SECP256K1] Increase precision of g1 and g2
Summary:
This allows us to shift by 256+128 = 384 bits, which is a multiple of the limb size of the scalar representation. This also happens to be the most precision possible for g2 that still fits into a 256-bit value.
Partial backport 2/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/76ed922a5f09d63e0622825ca83d9301c1ef3efe
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8038