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