add unordered parallel variants of (flat)traverse and (flat)sequence

[mberndt123]

Hi,

There currently doesn't seem to be a convenient way to traverse a Set with IO. traverse doesn't work because there's no Traverse instance for Set, and unorderedTraverse doesn't work because there is no CommutativeApplicative instance for IO. So I've added parUnorderedTraverse and parUnorderedSequence for that purpose.

I also added parUnorderedFlatTraverse and parUnorderedFlatSequence for symmetry, but given that the non-par versions of these don't exist I'm not really sure we need these.

Another logical set of operations would be parUnorderedTraverse_ and parUnorderedSequence_. But again, there is no unorderedTraverse_ or unorderedSequence_ in UnorderedFoldable, so it's not going to be any more efficient than the non-_ variants, and given that it's easy to just discard the return value I haven't added them.

So overall I think that parUnorderedTraverse and parUnorderedSequence should be pretty uncontroversial. Which of the other ones do you think we want? I think they all make sense for symmetry, but then I'm not sure how useful they are in practice.

Oh, and one more question: is FlatMap actually the right typeclass to use for the flat variants, or should it be CommutativeFlatMap?

[mberndt123]

OK, somebody beat me to it: #2931

Too bad I didn't notice that earlier. But hey, you probably really want those flat variants, don't you? 😆

[codecov-io]

Codecov Report

Merging #2946 into master will decrease coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2946      +/-   ##
==========================================
- Coverage   93.99%   93.84%   -0.15%     
==========================================
  Files         369      369              
  Lines        7056     7067      +11     
  Branches      179      195      +16     
==========================================
  Hits         6632     6632              
- Misses        424      435      +11

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/parallel.scala	62.5% <0%> (-17.5%)	⬇️
core/src/main/scala/cats/Parallel.scala	84.37% <0%> (-5.63%)	⬇️

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[LukaJCB]

Sorry for the wait, this looks great :)
Can we add some tests to the SyntaxSuite to ensure these resolve properly?

[mberndt123]

Hey @LukaJCB,

I've added some tests now. Thanks for the idea, it actually helped me shake out some silly mistakes in the implementation.

[mberndt123]

I've also removed ParallelUnorderedFlatTraverseOps and moved its only method to ParallelUnorderedTraverseOps. I don't think there's a reason to have separate classes for those two methods.

[LukaJCB]

Thanks, this looks great!