Add Coproduct fold

[peterneyens]

Add the fold operation for Coproduct mentioned in #987.
I am not sure how I could test this ?

[codecov-io]

Current coverage is 88.74%

Merging #1032 into master will decrease coverage by <.01%

1. 2 files (not in diff) in ...n/scala/cats/functor were modified. more
2. 2 files (not in diff) in .../src/main/scala/cats were modified. more

@@             master      #1032   diff @@
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  Files           215        215          
  Lines          2726       2727     +1   
  Methods        2664       2663     -1   
  Messages          0          0          
  Branches         57         59     +2   
==========================================
  Hits           2420       2420          
- Misses          306        307     +1   
  Partials          0          0          

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[ceedubs]

Thanks @peterneyens. I think this method is pretty much guaranteed to be implemented correctly if the types line up, so I'm not too worried about testing it. But it would also be nice to not have it reported as uncovered code. Would you be interested in writing an sbt-doctest example in the scaladoc? I think that would kind of knock out two birds with one stone, because it may be helpful for people and it should count as test coverage. You can could use simple NaturalTransformations such as the listToOption one that is used in NaturalTransformationTests.

[adelbertc]

👍

[ceedubs]

Awesome. Thanks!