Modify phutil_json_decode to use PhutilJSONParser.

Summary: Ref T5297. Modify phutil_json_decode to use PhutilJSONParser instead of the native json_decode.

Test Plan: Ran arc unit.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5297

Differential Revision: https://secure.phabricator.com/D9634