add syntax <&> for parallel

[jcouyang]

in #2038 introduced &> and <& syntax for parallel which are very helpful

why not have a <&> too, so I can

mab <&> ma

instead of

Parallel.parAp(mab)(ma)

[kailuowang]

Unfortunately, you can't add a method to a trait without breaking binary compatibility in Scala 2.11.
You need to add a new trait to add this method and have relevant objects extending it. And then you can have AllSyntaxBinCompat4 (a new trait introduced post 1.5.0 release) extend this.

[codecov-io]

Codecov Report

Merging #2671 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2671      +/-   ##
==========================================
+ Coverage   95.12%   95.12%   +<.01%     
==========================================
  Files         364      365       +1     
  Lines        6707     6753      +46     
  Branches      285      285              
==========================================
+ Hits         6380     6424      +44     
- Misses        327      329       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/parallel.scala	70.58% <0%> (-9.42%)	⬇️
...estkit/src/main/scala/cats/tests/ListWrapper.scala	100% <0%> (ø)

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[kubukoz]

I think you can add your syntax to the trait introduced here (in a similar way): #2600 (it was after 1.5.0)

[jcouyang]

thanks @kailuowang @kubukoz I added it to AllSyntaxBinCompat4 also the syntax object parallel
but I think the coverage drop because SyntaxSuite.scala is only compile time check, coverage report was not able picking that up
should I add a run time test case in ParallelSuite.scala?

[kubukoz]

I don't think anyone looks at the coverage reports, but if you want you can add a doctest to ensure this works as expected (I'm not sure syntax is being tested in ParallelSuite)

[jcouyang]

Good, I personally think the syntaxsuite is good enough for covering the syntax

[LukaJCB]

Looks good to me