Verify signatures after signing

[quad]

Description

Verify signatures after signing

As per BIP-340, footnote 14:

Verifying the signature before leaving the signer prevents random or attacker provoked computation errors. This prevents publishing invalid signatures which may leak information about the secret key. It is recommended, but can be omitted if the computation cost is prohibitive.

Notes to the reviewers

How do we test this?

Checklists

All Submissions:

• [ ] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing