ARROW-2650: [JS] Implement Unions

[trxcllnt]

Still need to add Unions to the integration tests, but I validated this locally with the JSON file from #987 and it seems to work:

$ ./bin/json-to-arrow.js -j ./test/data/json/union.json -a ./test/data/union-file.arrow 
$ cat ./test/data/union-file.arrow | ./bin/file-to-stream.js > ./test/data/union-stream.arrow
$ cat ./test/data/union-file.arrow | node ./targets/apache-arrow/bin/arrow2csv.js 
"row_id" | "union: Union<Int32 | Int64>"
       0 |                             0
       1 |                         [1,0]
       2 |                             2
       3 |                         [3,0]
       4 |                             4
       5 |                         [5,0]
       6 |                             6
       7 |                         [7,0]
       8 |                             8
       9 |                         [9,0]

$ cat ./test/data/union-stream.arrow | node ./targets/apache-arrow/bin/arrow2csv.js 
"row_id" | "union: Union<Int32 | Int64>"
       0 |                             0
       1 |                         [1,0]
       2 |                             2
       3 |                         [3,0]
       4 |                             4
       5 |                         [5,0]
       6 |                             6
       7 |                         [7,0]
       8 |                             8
       9 |                         [9,0]

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[wesm]

+1. For the record, I continue to be amazed that you're able to build this library effectively while writing few or no unit tests to accompany patches. I try my best not to lecture about software engineering, but without more tests it's going to be insanely risky to ever perform significant refactoring on this code. Any refactor could easily introduce unknown bugs.

[wesm]

Years ago I saw Uncle Bob's talk "What killed Smalltalk could kill Ruby, too" and I have taken the general idea to heart (without going full TDD; I tend to write unit tests while writing the implementation, rather than before): https://www.youtube.com/watch?v=YX3iRjKj7C0