Make functions tail-call recursive

[jfmengels]

2 changes here, both making some functions tail-call recursive.

For monotonePart, the |> prevented the optimization from happening.

For gatherWith, it's the same issue but because of <|. I made the same fix in elm-community/list-extra (elm-community/list-extra@2d0eb64#diff-97b2cb39788ae3d6877530635d216e6cd361006507af40173e5affcb5f095331)

I found this using using the [NoUnoptimizedRecursion](https://package.elm-lang.org/packages/jfmengels/elm-review-performance/latest/NoUnoptimizedRecursion] elm-review rule, which you can try out by running the following command:

elm-review --template jfmengels/elm-review-performance/example --rules NoUnoptimizedRecursion

There are some more to be fixed, but they're not as trivial, and the performance benefits will have to be evaluated.

PS: Running the tests doesn't work. The elm-explorations/test dependency is missing, and I think some others too, but I am not sure which one. NoRedInk/elm-plot-rouge?

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