Add Traverse[Id] and Id law-checking

[ceedubs]

We should also be able to add a Reducible instance for Id, but I think that should wait for #629.

[non]

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[codecov-io]

Current coverage is 76.43%

Merging #639 into master will increase coverage by +0.08% as of df5e6f8

@@            master    #639   diff @@
======================================
  Files          159     159       
  Stmts         2081    2084     +3
  Branches        70      70       
  Methods          0       0       
======================================
+ Hit           1589    1593     +4
  Partial          0       0       
+ Missed         492     491     -1

Review entire Coverage Diff as of df5e6f8

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[adelbertc]

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