Add foldlMaybe and code reuse

[theqp]

Description of the change

I created a function foldlMaybe because I noticed code duplication in minimumBy and maximumBy

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)