Add `fn zip_longest`

[TheLostLambda]

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There is a function version of zip_eq already, but it appears that zip_longest is still only available as a method?

Let me know if you think this is a worthwhile addition for completeness's sake, and I'll submit a PR!

[Philippe-Cholet]

I don't think we stand against adding new free functions, but we sure don't intend to be complete on that side.