Allow signing mutable data asynchronously

[BkPHcgQL3V]

This can be useful if we want to keep the private key out of the process.

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update [ ] Bug fix [X] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

If the sign field in opts has two arguments, assume the second one is for an asynchronous callback.

Restructure the code to accommodate this.

Which issue (if any) does this pull request address?

None.

Is there anything you'd like reviewers to focus on?

Maybe there is too much green for such a small addition.

[welcome[bot]]

🙌 Thanks for opening this pull request! You're awesome.