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