Add a context field to symbol objects
Summary:
See T1602.
This is just the minimal functional patch; the scripts will continue
working because of the DEFAULT ''.
Test Plan:
Can't fully test this until I get more code working, but
nothing broke horribly yet.
Reviewers: epriestley
Reviewed By: epriestley
CC: nh, aran, Korvin
Maniphest Tasks: T1602
Differential Revision: https://secure.phabricator.com/D3147