Add CKey::SignSchnorr and CPubKey::VerifySchnorr

Summary:
This is coded for 64-byte schnorr sigs (no flag byte).

Depends on D2345 and D2169 .

Test Plan:

• Tests added that mimic those used for SignECDSA/VerifyECDSA.
• Test that nonce reuse does not occur for ECDSA and Schnorr.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2348