Rename prod coproduct

[kailuowang]

fixes #1582
Note that this should be a fairly straightforward PR to review. However, github failed to render the diff in renamed files correctly - it displays the diff as a whole file removed and a new file. To overcome this, you might want to review the two commits separately.

[johnynek]

thanks for doing this! I think this is a win for consistency and readability prior to 1.0.

👍

[codecov-io]

Codecov Report

Merging #1589 into master will not change coverage.
The diff coverage is 98.87%.

@@           Coverage Diff           @@
##           master    #1589   +/-   ##
=======================================
  Coverage   92.42%   92.42%           
=======================================
  Files         249      249           
  Lines        3974     3974           
  Branches      131      131           
=======================================
  Hits         3673     3673           
  Misses        301      301

Impacted Files	Coverage Δ
free/src/main/scala/cats/free/Free.scala	88.23% <ø> (ø)	⬆️
core/src/main/scala/cats/data/Nested.scala	96.15% <ø> (ø)	⬆️
core/src/main/scala/cats/arrow/FunctionK.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/Inject.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/EitherK.scala	100% <100%> (ø)
core/src/main/scala/cats/data/Func.scala	100% <100%> (ø)	⬆️
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	89.83% <100%> (ø)	⬆️
core/src/main/scala/cats/syntax/eitherK.scala	100% <100%> (ø)
core/src/main/scala/cats/data/Tuple2K.scala	96.66% <96.66%> (ø)
... and 2 more

---

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[edmundnoble]

LGTM.

[adelbertc]

Bikeshedding, but Tuple2K feels weird even though it's correct since it's a higher-order Tuple2, but what about just TupleK ?

[kailuowang]

@adelbertc I don't dislike TupleK. I picked Tuple2K exactly due to the correctness, and TupleK sounds like it's arity free?

[adelbertc]

yeahhhhh i just don't like having numbers in names for some reason.. but oh well 👍

[edmundnoble]

@adelbertc I thought PairK might be nice for that reason, but that might drive people to use scala.Predef.Pair.