Adds test for Show[Const]

[mikejcurry]

Adds a test for Show[Const], includes:

• Simple test of a specific value
• Test that all output starts with “Const(“
• Test that the Show for the result of const.getConst is contained in the result
• Test that implicitly obtained Show instance produces a result consistent with direct method invocation.
• Tests that retagging the Const does not affect the Show result

The last one raises an interesting question. The phantom type of the Const might be interesting enough information in some cases. The question is whether it is interesting enough to show.

Basically, Const[Int, String] and Const[Int, Boolean] produce the same show result. I implemented a test for the behaviour primarily to demonstrate that it is the case, and to raise a discussion on whether it warrants inclusion.

[codecov-io]

Current coverage is 84.91%

Merging #704 into master will increase coverage by +0.13% as of df72291

@@            master    #704   diff @@
======================================
  Files          162     162       
  Stmts         2228    2228       
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1889    1892     +3
  Partial          0       0       
+ Missed         339     336     -3

Review entire Coverage Diff as of df72291

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[stew]

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[non]

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