[secp256k1] Implement Schnorr signatures

Summary:
This implement Schnorr signatures on secp256k1 as per https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki

This implement a variation of Schnorr similar to edDSA by Daniel J. Bernstein.

Test Plan: Added test cases for the signature scheme.

Reviewers: #bitcoin_abc, Fabien, jasonbcox, markblundeberg

Reviewed By: #bitcoin_abc, Fabien, jasonbcox, markblundeberg

Subscribers: jasonbcox, markblundeberg, Fabien, teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2169