Backport bifold in Bifoldable typeclass to scala_2.11

[gagandeepkalra]

addresses #3143

[gagandeepkalra]

I was hoping to augment this PR with another Bifoldable law, but couldn't get around binary compatibility. I settled with a doctest for now.

[rossabaker]

Thanks! A couple consistency nits, but the substance looks good.

[codecov-io]

Codecov Report

Merging #3242 into scala_2.11 will increase coverage by <.01%.
The diff coverage is 100%.

@@              Coverage Diff               @@
##           scala_2.11    #3242      +/-   ##
==============================================
+ Coverage        93.5%   93.51%   +<.01%     
==============================================
  Files             383      384       +1     
  Lines            7149     7151       +2     
  Branches          193      192       -1     
==============================================
+ Hits             6685     6687       +2     
  Misses            464      464

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/bifoldable.scala	100% <100%> (ø)

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[rossabaker]

My apologies. I overlooked the update to this. 👍