Make supsubs work correctly with character boxes

Summary:
In rule 18a. of the TeX book, it says we're supposed to set `u` and `v` to 0 if
the nucleus is a character box. Character boxes are loosely defined, but through
experimentation they are anything that contains a single character, including
ordgroups with a single element in them. This change makes KaTeX follow this
rule, and fixes T2404.

Test Plan:
- Make sure the new huxley screenshot looks good
- Make sure none of the huxley tests have changed

Reviewers: alpert

Reviewed By: alpert

Maniphest Tasks: T2404

Differential Revision: http://phabricator.khanacademy.org/D8275

test/huxley/Huxleyfile

@@ -42,3 +42,6 @@ url=http://localhost:7936/test/huxley/test.html?m=\KaTeX
 
 [RlapBug]
 url=http://localhost:7936/test/huxley/test.html?m=\frac{\rlap{x}}{2}
+
+[SupSubCharacterBox]
+url=http://localhost:7936/test/huxley/test.html?m=a_2f_2{f}_2{aa}_2{af}_2

test/huxley/SupSubCharacterBox.huxley/record.json

@@ -0,0 +1 @@
+{"py/object": "huxley.run.Test", "screen_size": {"py/tuple": [1024, 768]}, "steps": [{"py/object": "huxley.steps.ScreenshotTestStep", "index": 0, "offset_time": 0}]}