Reinstate @johnynek's intercalate optimization from #3279

[travisbrown]

This is a minor thing but I'm opening this PR so we don't lose track of it.

@johnynek originally included this optimization in #3279, but removed it because prior to #3303 combine for BigDecimal wasn't actually commutative on Scala 2.13. That's fixed now, so we can put this back.

[codecov-io]

Codecov Report

Merging #3321 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3321      +/-   ##
==========================================
+ Coverage   93.31%   93.31%   +<.01%     
==========================================
  Files         378      378              
  Lines        7684     7686       +2     
  Branches      233      222      -11     
==========================================
+ Hits         7170     7172       +2     
  Misses        514      514

Flag	Coverage Δ
#scala_version_212	93.38% <100%> (ø)	⬆️
#scala_version_213	93.09% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
.../main/scala/cats/kernel/CommutativeSemigroup.scala	66.66% <100%> (+16.66%)	⬆️

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[travisbrown]

This commit is merged but for some reason the PR didn't close itself. I'm leaving it open for now in the hopes that the issue will resolve itself (and keep our PR history accurate), but please don't merge again.