Test Monad map/flatMap coherence law

[ceedubs]

This is basically the same story as #728, but it was introduced in
fa6457a.

[codecov-io]

Current coverage is 85.86%

Merging #736 into master will increase coverage by +0.05% as of a0cb9e2

@@            master    #736   diff @@
======================================
  Files          162     162       
  Stmts         2249    2250     +1
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1930    1932     +2
  Partial          0       0       
+ Missed         319     318     -1

Review entire Coverage Diff as of a0cb9e2

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[stew]

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