Abstract out challenge computation in schnorrsig

Summary:
This is a partial backport of secp256k1 PR808 : https://github.com/bitcoin-core/secp256k1/pull/808/commits/87af00b511f2938b6b4799f94d446a005730515e

Depends on D7665

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7666