Fixes and tests for pre-signing and signing protocols

[davidsemakula]

Summary of changes Changes introduced in this pull request:

• Fixes pre-signing including support for threshold signing (based on GG18 and GG20 as recommended in Section 1.2.8 of CGGMP20, see inline comments and tests for details)
• Fixes signing
• Adds tests for pre-signing and signing

NOTE: No changes and/or tests were made/added for related identifiable aborts so those may still contain errors.

Reference issue to close (if applicable)

Closes

[drewstone]

Awesome work @davidsemakula, thanks for your contribution. Will merge once CI passes.

[davidsemakula]

@drewstone thanks 🙂. Also Just FYI, current CI failures seem unrelated to/and existed before my changes (that's my understanding from looking at this, this and this, it also looks the like the most recently merged PRs didn't pass CI either) - there were a few rustfmt failures from my changes but I believe I've fixed those with the last 2 "formatting" commits.

In general running cargo fmt makes a lot of changes to the code base that are unrelated to the changes in this PR so I was hesitant to run it on the whole project as it would make this PR harder to review.

I have all tests running successfully locally, so given the above issues, I don't think I'm best placed to fix the CI issues in this case 🙂.