fix: limb range check for fpelem

[mrain]

closes: #XXXX

This PR:

Missing limb range check when creating FpElemVar leads to fiat shamir soundness break.
This PR fixes it, with regression tests.

This PR does not:

Key places to review:

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• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added relevant changelog entries to the CHANGELOG.md of touched crates.
• Re-reviewed Files changed in the GitHub PR explorer

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