Add bifold to Bifoldable close #2947.

[takayahilton]

No description provided.

[kailuowang]

We can't break BC yet, but we should be able to allow this soon, so I scheduled it to 2.1. Also it would be nice to have some tests.

[codecov-io]

Codecov Report

Merging #2962 into master will decrease coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2962      +/-   ##
==========================================
- Coverage   93.99%   93.92%   -0.07%     
==========================================
  Files         369      371       +2     
  Lines        7056     7062       +6     
  Branches      179      193      +14     
==========================================
+ Hits         6632     6633       +1     
- Misses        424      429       +5

Impacted Files	Coverage Δ
core/src/main/scala/cats/Bifoldable.scala	83.33% <0%> (-7.58%)	⬇️
core/src/main/scala/cats/syntax/option.scala	100% <0%> (ø)	⬆️
testkit/src/main/scala/cats/tests/CatsSuite.scala	33.33% <0%> (ø)
...stkit/src/main/scala/cats/tests/CatsEquality.scala	66.66% <0%> (ø)

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[travisbrown]

I just noticed this PR, which was also done in #3088, which was just merged. I'm going to close this one, but thanks, @takayahilton, and I'm sorry we didn't see this one.

I'm not really sure the test here is needed, since it's just checking the default implementation, and I wouldn't really expect Bifoldable implementers to override this method.