Add law-checking for Either Eq instance

[ceedubs]

I think we were trying to do this before, but the approach we were
taking was testing the Order instance instead of the Eq instance.

[non]

👍

[codecov-io]

Current coverage is 83.08%

Merging #681 into master will increase coverage by +0.14% as of 4408039

@@            master    #681   diff @@
======================================
  Files          162     162       
  Stmts         2217    2217       
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1839    1842     +3
  Partial          0       0       
+ Missed         378     375     -3

Review entire Coverage Diff as of 4408039

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[adelbertc]

👍