Fs2 EncodeStream for both ConcurrentEffect and Effect monads

[joroKr21]

The ConcurrentEffect instances are prioritized higher.
This means users can benefit from cancellation when possible
without loss of generality.

@vkostyukov this is what I had in mind with providing both versions and prioritizing them. But I don't know how to test that cancellation actually works. Any ideas?

[joroKr21]

It works just fine on 2.12 😞

[codecov-io]

Codecov Report

Merging #1082 into master will decrease coverage by 0.47%.
The diff coverage is 52.63%.

@@            Coverage Diff             @@
##           master    #1082      +/-   ##
==========================================
- Coverage   80.62%   80.15%   -0.48%     
==========================================
  Files          54       54              
  Lines        1017     1028      +11     
  Branches       59       59              
==========================================
+ Hits          820      824       +4     
- Misses        197      204       +7

Impacted Files	Coverage Δ
fs2/src/main/scala/io/finch/fs2/package.scala	59.25% <52.63%> (-9.5%)	⬇️
core/src/main/scala/io/finch/Endpoint.scala	72.41% <0%> (-0.58%)	⬇️

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[vkostyukov]

I like this @joroKr21! Does this slow down compilation at all? We've been trying to be careful with introducing more complexity that could potentially slow down the implicit resolution.

@rpless @sergeykolbasov Mind taking a look?

[joroKr21]

I'm not too concerned about compile times. After scala/scala#7067 implicits in Scala 2.12 and 2.13 are not that expensive anymore.

I'm more concerned about:

1. How to test cancellation actually works?
2. How to document the behavior (although that not be as bad if we think about this as an optimization).

[vkostyukov]

@joroKr21 Mind rebasing this?

[sergeykolbasov]

👍

I just solved the Fiber and bracketCase for stream cancellation on my own and then re-checked this PR. Oops 😄

On cancellation test topic, could we get some inspiration from cats-effect laws: https://github.com/typelevel/cats-effect/blob/master/laws/shared/src/test/scala/cats/effect/IOCancelableTests.scala#L76-L89 ?

[vkostyukov]

FWIW, I'm happy if we decide to add a specific cancelation test later, in the upcoming commit. Let's merge this, it's been on review for quite some time.

[joroKr21]

On cancellation test topic, could we get some inspiration from cats-effect laws: https://github.com/typelevel/cats-effect/blob/master/laws/shared/src/test/scala/cats/effect/IOCancelableTests.scala#L76-L89 ?

First we have to migrate the tests to use a TestContext which can tick.