Add IO implementation of parMap

[pakoito]

First pass at parallelising IO.

TODO:

• Nice API
• Tests
• Docs

[codecov]

Codecov Report

Merging #950 into master will increase coverage by 0.37%.
The diff coverage is 67.92%.

@@             Coverage Diff              @@
##             master     #950      +/-   ##
============================================
+ Coverage     46.21%   46.59%   +0.37%     
- Complexity      642      643       +1     
============================================
  Files           304      306       +2     
  Lines          7859     7965     +106     
  Branches        806      816      +10     
============================================
+ Hits           3632     3711      +79     
- Misses         3906     3927      +21     
- Partials        321      327       +6

Impacted Files	Coverage Δ	Complexity Δ
.../arrow-effects/src/main/kotlin/arrow/effects/IO.kt	70% <100%> (+3.89%)	8 <0> (ø)	⬇️
...ffects/src/main/kotlin/arrow/effects/IOParallel.kt	20.68% <20.68%> (ø)	0 <0> (?)
...ain/kotlin/arrow/effects/internal/ParallelUtils.kt	85.52% <85.52%> (ø)	0 <0> (?)
...effects/src/main/kotlin/arrow/effects/IORunLoop.kt	73.36% <0%> (+1.08%)	43% <0%> (+1%)	⬆️
...fects/src/main/kotlin/arrow/effects/IOInstances.kt	50% <0%> (+4.16%)	0% <0%> (ø)	⬇️
...ts/src/main/kotlin/arrow/effects/internal/Utils.kt	80% <0%> (+8%)	0% <0%> (ø)	⬇️

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[pakoito]

@raulraja I'm happy to merge without docs for now, to validate our assumptions of how it should work and announce it later.

[raulraja]

Looks great, awesome work!. Instead of creating the arities for mapN in the companion shouldn't these be in the type classes and exported with the work Eugenio is doing for type class extension generation?

[pakoito]

@raulraja It's IO-exclusive for now, it isn't generalised to run for any Effect because it uses unsafeRunAsync().

[pakoito]

@raulraja I've generalised for Effect, although it's a bit ugly because we don't have a nice Future box where to put the "starters" in.