ARROW-12493: Add support for writing dictionary arrays to CSV and JSON

[tustvold]

Provide support for serializing dictionary arrays to CSV and JSON by hydrating them to their underlying representation. This is not the most efficient way to do this, but was the simplest way I could think of to cover all bases.

It may be worthwhile special-casing StringDictionaries with a more efficient implementation in a subsequent PR, as I imagine they're the most common form of DictionaryArray.

[codecov-commenter]

Codecov Report

Merging #16 (4aaa465) into master (5479e19) will increase coverage by 0.01%.
The diff coverage is 97.29%.

❗ Current head 4aaa465 differs from pull request most recent head 4fa2a1c. Consider uploading reports for the commit 4fa2a1c to get more accurate results

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   82.47%   82.48%   +0.01%     
==========================================
  Files         162      162              
  Lines       43414    43447      +33     
==========================================
+ Hits        35806    35838      +32     
- Misses       7608     7609       +1     

Impacted Files	Coverage Δ
arrow/src/csv/writer.rs	83.01% <91.66%> (+0.27%)	⬆️
arrow/src/json/writer.rs	91.92% <100.00%> (+0.43%)	⬆️

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[alamb]

Looks good to me. Thank you @tustvold

[alamb]

🧁

[jorgecarleitao]

We had to perform a small re-write of master. The commits may look a bit odd, but it should not cause conflicts. Could you kindly rebase this against the latest master to make it easier to review?