Add par functions for UnorderedTraverse.

[takayahilton]

Added parUnordered.

[codecov-io]

Codecov Report

Merging #2931 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2931      +/-   ##
==========================================
+ Coverage   94.03%   94.03%   +<.01%     
==========================================
  Files         370      370              
  Lines        7038     7046       +8     
  Branches      186      188       +2     
==========================================
+ Hits         6618     6626       +8     
  Misses        420      420

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/CatsSuite.scala	33.33% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/parallel.scala	82.75% <100%> (+2.75%)	⬆️
core/src/main/scala/cats/Parallel.scala	90.62% <100%> (+0.62%)	⬆️

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[mberndt123]

@LukaJCB,

Hey, I just implemented the same thing. Unfortunately I only noticed this PR after submitting my PR #2946. However my PR also includes parUnorderedFlatTraverse and parUnorderedFlatSequence, so if you think we need those, you can merge my PR instead. If you feel they're not needed, you can just close my PR.

[kailuowang]

close since #2946 is merged.