Render expressions like 1/3 * 6/8 nicely

Summary:
Previously, "1/3 * 6/8" would be displayed as

`\frac{\frac{1}{3} \cdot 6}{8}`

this commit makes it render as

`\frac{1}{3} \cdot \frac{6}{8}`.

A consequence of this is that expressions like `1*2*3/4` now render as
`1*2*(3/4)`.

Also fixes a bug caused by a misuse of splice.

Test Plan:
  * Open test.html
  * Verify all tests pass.

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D6027