Parse metrics directly from .ttf files

Summary:
Fixes T1325.

This has the advantage of not having to do the mapping manually. Also it means that characters like \neq are included in the mapping, so this resolves T1323 as well.

I opted to group the metrics by character instead of by metric type because I think it'll gzip better and it makes getCharacterMetrics simpler anyway.

Test Plan: The test page looks identical to the pixel to my eye.

Reviewers: emily

Reviewed By: emily

Maniphest Tasks: T1323, T1325

Differential Revision: http://phabricator.khanacademy.org/D3535