Add .parFlatTraverse and .parFlatSequence syntax

[Avasil]

Fixes #2255.

[codecov-io]

Codecov Report

Merging #2256 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2256      +/-   ##
==========================================
+ Coverage   94.96%   94.96%   +<.01%     
==========================================
  Files         334      334              
  Lines        5801     5805       +4     
  Branches      214      218       +4     
==========================================
+ Hits         5509     5513       +4     
  Misses        292      292

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/parallel.scala	72.72% <100%> (+15.58%)	⬆️

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[Avasil]

@LukaJCB @kailuowang
Could you please review/approve it again? I had to resolve conflicts with master which dismissed reviews. Thanks!