schnorrsig: Add BIP-340 nonce function

Summary:
This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/7332d2db6b62fda851f9ed8adbfda187a875b84e

Depends on D7646

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7647