Add tests of kleisli left and right identity laws

[ceedubs]

cc @fthomas :)

[fthomas]

👍

That should be the last one, right? :-)

[ceedubs]

Yes, I think so! :)

[codecov-io]

Current coverage is 87.17%

Merging #748 into master will increase coverage by +0.08% as of b7b7048

@@            master    #748   diff @@
======================================
  Files          166     166       
  Stmts         2293    2293       
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1997    1999     +2
  Partial          0       0       
+ Missed         296     294     -2

Review entire Coverage Diff as of b7b7048

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