Better Chain Arbitrary

[LukaJCB]

This should fix the concerns @johnynek had in #2371

Also optimizes the Chain.apply builder :)

[LukaJCB]

Seems it has already caught at least one bug 😄

[LukaJCB]

The small bug in uncons has been fixed, thanks a lot to @johnynek for pushing for the better arbitrary instance :)

[codecov-io]

Codecov Report

Merging #2420 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2420      +/-   ##
==========================================
+ Coverage    95.2%   95.22%   +0.01%     
==========================================
  Files         351      351              
  Lines        6366     6368       +2     
  Branches      280      279       -1     
==========================================
+ Hits         6061     6064       +3     
+ Misses        305      304       -1

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Chain.scala	97.42% <100%> (+0.32%)	⬆️
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	99.17% <100%> (+0.07%)	⬆️

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[kailuowang]

Nice!

[johnynek]

Thanks for taking!

[LukaJCB]

Merging with two sign-offs :)

[ceedubs]

Sorry I didn't get to the review @LukaJCB; I was on a plane. I'll submit some PRs that I crafted up while I was on it though :)