Add a native JSON parser.

Summary: Ref T5297. Add [[https://github.com/Seldaek/jsonlint | JsonLint]] as an external library and provide `PhutilJSONParser` as a wrapper around the library's functionality.

Test Plan: Wrote some basic unit tests and ran `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5297

Differential Revision: https://secure.phabricator.com/D9622