Monad writer, fixes #889

[adelbertc]

Other instances are possible: https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Writer-Class.html

But I realized we also don't have similar instances for the other MTL type classes, so I'll add it all in one fell swoop

[codecov-io]

Current coverage is 88.90%

Merging #911 into master will decrease coverage by -0.69% as of cc24bac

@@            master   #911   diff @@
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  Files          181    182     +1
  Stmts         2423   2155   -268
  Branches        76     42    -34
  Methods          0      0       
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- Hit           2171   1916   -255
  Partial          0      0       
+ Missed         252    239    -13

Review entire Coverage Diff as of cc24bac

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[stew]

👍

[ceedubs]

In several places this has (A, W), but (W, A) seems like it would line up more nicely with our WriterT. Is there any particular reason you went with the former?

Do people have experience using listen and listens? I will admit to never using MonadWriter, but it seems to me like working with the tuples could be a bit of a hassle. I find myself wondering if it would often be simpler to discard the A component and let people use flatMap and then a method like these if they want a reference to the A component. Just a brain dump: I'm curious about the experience of people who have actually used MonadWriter.

[adelbertc]

So I copied it from https://github.com/purescript/purescript-transformers/blob/master/docs/Control/Monad/Writer/Class.md - I think it uses tuples instead of separate args because the underlying representation for Writer is a tuple.

[non]

Looks good to me. 👍