Add a simple symbol lookup interface for cross-references

Summary: This will get fancier, but here's a basic interface for doing symbol
lookups. Still all pretty tentative.

Test Plan: Looked up various things, got some sensible results.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: tuomaspelkonen

CC: aran, tuomaspelkonen

Differential Revision: 900