feat(identity): make secp256k1::SecretKey::sign infallible

[thomaseizinger]

Description

Previously, this function would return an error in case the provided digest is not of 32 bytes long. As per our spec, we hash all messages with SHA256 before signing, thus this error can never happen in practice.

This brings us one step closer to an infallible Keypair::sign which is now only fallible due to RSA signing. If we manage to fix that as well, constructors like noise::Config::new will become infallible.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mergify]

This pull request has merge conflicts. Could you please resolve them @thomaseizinger? 🙏