Make json_writer a structured_writer, improve output formatting

[WardBrian]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

Follow up on #3191.

This PR:

• Improves the output of the JSON writer to be a bit more human readable (mainly by more consistently outputting newlines between key:value pairs)
• Updates both structured_writer and json_writer such that the latter can be a subclass of the former.
• Fixed a bug where j.begin_record("foo"); j.end_record(); j.write("anything here"); would generate invalid JSON

Intended Effect

How to Verify

Side Effects

Documentation

Copyright and Licensing

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By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)