refactor: simplify k generation following RFC 6979

[tcoratger]

Summary

This pull request introduces a direct integration of the rfc6979 crate to generate the scalar k according to RFC 6979. The integration is aimed at streamlining the logic within our codebase, ensuring simplicity, and reducing maintenance overheads by relying on established libraries from RustCrypto.

Changes Made

• Integrated the rfc6979 crate to handle the generation of scalar k.
• Refactored relevant sections of the codebase to utilize the new integration.
• Ensured consistency and compliance with RFC 6979 standards.

Motivation

• Simplifying Codebase: By directly leveraging the rfc6979 crate, we simplify the logic related to k generation, making it more concise and maintainable.
• Reduced Maintenance Burden: Depending on established libraries from RustCrypto reduces the need for maintaining custom solutions in our repository, ensuring compatibility and reliability.

[xJonathanLEI]

Do we have to use prerelease versions? I don't think it's the best idea to depend on prereleases. If we depend on upcoming features then we can wait for a release.

[tcoratger]

Yes in order to simplify the code of generate_k and avoid doing some manual stuffs, we need to depend on this up to date version. But rfc6979 is a crate of RustCrypto which is the official Rust crypto library. It is audited and constantly checked and updated. The changes are minor between versions so I don't think there is any risk in doing this. Precisely, the version previously used was revised and corrected later (if I'm not mistaken it dated 3 years ago, which isn't necessarily great either).

[xJonathanLEI]

rfc6979 v0.4.0 was out 2023-03-01 tho.

[tcoratger]

Yes but I checked when I did the manipulations and what is done here was impossible with this version, meaning that there was a fix in the middle. Feel free to wait for an update is needed but I think this is completely fine to do this with beta version.

[xJonathanLEI]

Yeah I understand that this is a new feature not available from v0.4.0. That said, I prefer to wait for the release, as we're actually not getting much out of it beyond "our code now looks nicer", which is a nice-to-have that we don't have to rush for IMO.

[xJonathanLEI]

Rebased to keep branch from becoming staled.