Add \scriptspace after superscripts and subscripts

Summary: Plain TeX assigns `\scriptspace=0.5pt`, which is used in Rules 18bcd in the TeXbook's Appendix G.

Test Plan:
- Look at `x^{x^{x_{x_{x_{x_x}}}}}\biggr|`, see a gap that looks similar to the one produced by real TeX.
- Look at the huxley tests, see that they look reasonable. Make sure that all changes to other huxley tests are now correct, and only changed because of this.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D12763

test/huxley/Huxleyfile.json

@@ -117,5 +117,11 @@
         "name": "VerticalSpacing",
         "screenSize": [1024, 768],
         "url": "http://localhost:7936/test/huxley/test.html?pre=potato<br>blah&post=<br>moo&m=x^{\\Huge y}z"
+    },
+
+    {
+        "name": "SupSubHorizSpacing",
+        "screenSize": [1024, 768],
+        "url": "http://localhost:7936/test/huxley/test.html?m=x^{x^{x}}\\Big|x_{x_{x_{x_{x}}}}\\bigg|x^{x^{x_{x_{x_{x_{x}}}}}}\\bigg|"
     }
 ]