Add Representable instance for Eval

[denisrosca]

Closes #2320

[denisrosca]

Reopening to trigger CI build

[codecov-io]

Codecov Report

Merging #2329 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2329      +/-   ##
==========================================
+ Coverage   95.07%   95.08%   +<.01%     
==========================================
  Files         343      343              
  Lines        5931     5935       +4     
  Branches      217      211       -6     
==========================================
+ Hits         5639     5643       +4     
  Misses        292      292

Impacted Files	Coverage Δ
core/src/main/scala/cats/Eval.scala	98.8% <100%> (+0.05%)	⬆️

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