Add WriterT instances for supertypes of Monad

[ceedubs]

This is a partial fix for #620. There are still more WriterT instances that could be added, but this is already a sizable PR, so I thought getting in the supertypes of Monad would be a good chunk of work to split off.

I've gone to fairly great lengths to ensure that implicit resolution works in various cases in the tests. Perhaps I've even gone a bit overboard, but I think that these tests are kind of necessary, because there are some inference issues with Id that are compounded by issues with type aliases. In fact I had to rework the prioritization hierarchy several times as these tests uncovered issues.

[codecov-io]

Current coverage is 76.35%

Merging #628 into master will increase coverage by +0.22% as of bc03128

@@            master    #628   diff @@
======================================
  Files          159     159       
  Stmts         2070    2085    +15
  Branches        69      69       
  Methods          0       0       
======================================
+ Hit           1576    1592    +16
  Partial          0       0       
+ Missed         494     493     -1

Review entire Coverage Diff as of bc03128

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[non]

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[stew]

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