Add CancellableEffect typeclass. Use it to give races to IO

[pakoito]

This PR races two IOs, allowing cancellation between them.

It also adds the typeclass CancellableEffect, that was needed to achieve this.

[codecov]

Codecov Report

Merging #984 into master will decrease coverage by 0.63%.
The diff coverage is 43.95%.

@@             Coverage Diff             @@
##             master    #984      +/-   ##
===========================================
- Coverage     47.03%   46.4%   -0.64%     
+ Complexity      694     692       -2     
===========================================
  Files           320     320              
  Lines          8091    8255     +164     
  Branches        848     859      +11     
===========================================
+ Hits           3806    3831      +25     
- Misses         3942    4080     +138     
- Partials        343     344       +1

Impacted Files	Coverage Δ	Complexity Δ
...s-rx2/src/main/kotlin/arrow/effects/ObservableK.kt	61.53% <0%> (-6.55%)	9 <0> (ø)
...cts-rx2/src/main/kotlin/arrow/effects/FlowableK.kt	50.63% <0%> (-3.43%)	9 <0> (ø)
...fects-rx2/src/main/kotlin/arrow/effects/SingleK.kt	73.33% <0%> (-14.67%)	6 <0> (ø)
...-test/src/main/kotlin/arrow/test/laws/MonadLaws.kt	48.33% <0%> (ø)	12 <1> (ø)	⬇️
...rc/main/kotlin/arrow/effects/FlowableKInstances.kt	57.14% <0%> (-2.12%)	0 <0> (ø)
.../src/main/kotlin/arrow/effects/SingleKInstances.kt	70% <0%> (-3.69%)	0 <0> (ø)
.../main/kotlin/arrow/effects/ObservableKInstances.kt	55.55% <0%> (-2.14%)	0 <0> (ø)
...rc/main/kotlin/arrow/effects/DeferredKInstances.kt	68.18% <0%> (-3.25%)	0 <0> (ø)
...effects/src/main/kotlin/arrow/effects/IORunLoop.kt	73.79% <100%> (-1.07%)	45 <0> (-2)
...w/effects/data/internal/IOCancellationException.kt	100% <100%> (ø)	1 <1> (ø)	⬇️
... and 12 more

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[pakoito]

@raulraja ready for review!

[raulraja]

Hoping these would make it to a Concurrent type class in the near future.

[raulraja]

I believe this should be just Concurrent and include the race style algebra combinators in this type class so they are available for all concurrent effects in the spirit that concurrent effects are cancelable. For what is worth in cats we have:

Concurrent[F] extends Async[F]
ConcurrentEffect[F] extends Concurrent[F] with Effect[F]

[raulraja]

This is definitely an improvement from what we have but I'm missing the type class aspect of race, etc.. which should live in my opinion in Concurrent just above Async in a similar way to how it's done in cats-effect