Make boilerplate syntax classes extend Serializable

[ceedubs]

Today I ran into a serializability issue when using mapN in a Spark
environment 😬.

[codecov-commenter]

Codecov Report

Merging #3439 into master will decrease coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3439      +/-   ##
==========================================
- Coverage   91.64%   91.43%   -0.21%     
==========================================
  Files         381      380       -1     
  Lines        8269     8244      -25     
  Branches      227      227              
==========================================
- Hits         7578     7538      -40     
- Misses        691      706      +15     

Flag	Coverage Δ
#scala_version_212	?
#scala_version_213	91.43% <ø> (ø)

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/semigroupal.scala	50.00% <ø> (ø)
core/src/main/scala/cats/data/OneAnd.scala	79.10% <0.00%> (-16.49%)	⬇️
core/src/main/scala/cats/Reducible.scala	95.65% <0.00%> (-2.18%)	⬇️
core/src/main/scala/cats/syntax/either.scala	81.61% <0.00%> (-1.72%)	⬇️
...rc/main/scala/cats/laws/discipline/arbitrary.scala	98.29% <0.00%> (-0.86%)	⬇️
core/src/main/scala/cats/Show.scala	67.56% <0.00%> (-0.86%)	⬇️
core/src/main/scala/cats/instances/try.scala	86.44% <0.00%> (-0.45%)	⬇️
core/src/main/scala/cats/data/Tuple2K.scala	91.30% <0.00%> (-0.19%)	⬇️
.../src/main/scala/cats/laws/discipline/MiniInt.scala	97.22% <0.00%> (-0.08%)	⬇️
core/src/main/scala/cats/data/NonEmptyChain.scala	92.92% <0.00%> (-0.08%)	⬇️
... and 10 more

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[travisbrown]

Definite 👍 to the boilerplate additions. For SemigroupalOps, though, wouldn't it be better to do this at the Simulacrum level, for all Ops traits? I've just done that in #3440 via this Simulacrum Scalafix change.

[travisbrown]

By the way if anyone else is annoyed by scrolling past the huge Codecov comment in PRs like this, #3428 fixes that, and just needs one more review to merge.

[djspiewak]

in a Spark environment

Pretty sure I just diagnosed the problem here. ;-)

[travisbrown]

Merging this now. I'll include changing back the SemigroupalOps addition when I rebase #3440.