feat: memoize Signature polynomial decoding

[austinabell]

Describe your changes

Was reading code and saw this TODO to memoize, assumed this would be what was intended.

There was a similar one for the KeyPair public key commitment, but I didn't include with these as it would remove the Copy derivation, and I wanted these changes to be less opinionated.

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.

[sonarqubecloud]

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[bobbinth]

Looks great! Thank you for fixing this! (and sorry for such a long review delay - I just got back from devconnect).