Add some tests for Coproduct and WriterT

[adelbertc]

No description provided.

[codecov-io]

Current coverage is 89.49%

Merging #866 into master will increase coverage by +0.21% as of 6c2cd6f

@@            master    #866   diff @@
======================================
  Files          168     168       
  Stmts         2323    2323       
  Branches        75      75       
  Methods          0       0       
======================================
+ Hit           2074    2079     +5
  Partial          0       0       
+ Missed         249     244     -5

Review entire Coverage Diff as of 6c2cd6f

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[ceedubs]

👍

[stew]

👍