Fix Vec/String warnings

[hackaugusto]

Describe your changes

Like the changes in crypto, this requires the changes in winterfell to add the boxed mod.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[hackaugusto]

Thank you for the review! I'm going to mark this as a draft so we don't merge it before winterfell :)