Add parallel instance for IorT

[andyscott]

The same functionality as #2059, but for IorT instead of Ior.

[codecov-io]

Codecov Report

Merging #2060 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2060      +/-   ##
==========================================
- Coverage   94.66%   94.65%   -0.02%     
==========================================
  Files         318      318              
  Lines        5380     5405      +25     
  Branches      112      199      +87     
==========================================
+ Hits         5093     5116      +23     
- Misses        287      289       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/IorT.scala	97.76% <100%> (+0.34%)	⬆️
...main/scala/cats/kernel/laws/PartialOrderLaws.scala	90.9% <0%> (-9.1%)	⬇️
...c/main/scala/cats/laws/discipline/ArrowTests.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/Invariant.scala	95.23% <0%> (ø)	⬆️
laws/src/main/scala/cats/laws/MonadErrorLaws.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/instances/function.scala	92.3% <0%> (ø)	⬆️
core/src/main/scala/cats/MonadError.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/SemigroupK.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Ior.scala	100% <0%> (ø)	⬆️
... and 9 more

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[LukaJCB]

This is really cool, but I think it'd be even better, if we could add an extra instance that uses Parallel on the F if it exists (this is what I meant yesterday). So something like:

implicit def catsDataParallelForParallelIorT[M[_], F[_], E](implicit P: Parallel[M, F], E: Semigroup[E]) = ...

We can merge it like this though and I could add it later :)

[kailuowang]

just realized that this one is bin compat, so we can potentially wait after 1.0 but obviously there is no reasons that we have to.

[kailuowang]

@LukaJCB shall we merge this for now and add that instance in a followup (if you do merge please change the milestone to 1.0) thanks!

[andyscott]

@LukaJCB -- Got it. I'm adding a higher priority instance that's parallel over the effect as well.

[LukaJCB]

Thanks again!