feat(schnorr): add support for schnorr signatures

[kewde]

I won't be continuing this work but it's a good base for anyone else.

• [ ] Needs a fix for elliptic - reject overflowing secret keys.
• [ ] Only on test vector was ported - should be all of them.
• [ ] Browser fallback for schnorr with elliptic is not included in this PR.

helps solve #164