Add schnorr sign benchmark

Summary:
As per title.

Depends on D2747.

Test Plan: ./bench_sign

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, jasonbcox

Subscribers: jasonbcox, Fabien, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2748