Move some utils from libiop to libff

[alexander-zw]

Description

closes: scipr-lab/libiop#42

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• [ √ ] Targeted PR against correct branch (develop)
• [ √ ] 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 a relevant changelog entry to the Unreleased section in CHANGELOG.md
• [ √ ] Re-reviewed Files changed in the Github PR explorer

[ValarDragon]

LGTM sans minor comment for adapting power(exponent unsigned long)!