Add special grouping functions

[Martoon-00]

Description

I often witnessed the need in [(a, b)] -> [(a, [b])] grouping function, so adding it here since we are working on similar things in-parallel.

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[Martoon-00]

Additionally, I'd like to include groupByKeyOn—it's less general, but it avoids those explicit preconditions by leaning on Eq.

I also like how *On version would go without preconditions on the provided function.

My thought on not including it was that other packages seem to avoid the addition of *On version unless it does something special compared to passing mere on call (i.e. there is no groupOn f = groupBy ((==) 'on' f)).

Other examples: sortOn which has slightly different performance characteristics and non-existing Data.List.NonEmpty.groupOn.

So I think if we go with adding groupByKeyOn, we should be consistent and add mere groupOn too, but this is likely the work for a separate issue.

What do you think?

[treeowl]

I would like to have it for all the things. Sorting is weird because of sortOn f vs. sortBy (compare `on` f). I don't know if any of the other common functions have both options. I don't think these ones do.

[Martoon-00]

Okay, added On versions for grouping, and sorting is out of scope of this issue.

@treeowl Requesting the final review :eyes: