* Padding over \sqrt and Paths for frac-line
This replaces two earlier PRs.
* Restore reaction arrows
* regenerate screenshots
* Set line padding in a constant
* Update with latest master
* Fix lint error
* update screenshots
* Add \smash, laps, spaces, and phantoms
1. Support `\smash`, including the optional argument from AMS.
2. Change `\llap` and `\rlap` so that they render in text style. Repeat: This *changes* KaTeX behavior.
3. Add `\mathllap` and `\mathrlap`. These will act as they do in `mathtools` and as in previous KaTeX versions of `\llap` and `\rlap`.
4. Add `\mathclap` and `\clap`.
5. Add `\hphantom` and \vphantom`.
6. Add `\thinspace`, `\medspace`, `\thickspace`
7. Add `\hspace`.
This work will resolve issue #270 and parts of #50 and #164.
A. Perlis has written a [concise description](https://www.math.lsu.edu/~aperlis/publications/mathclap/perlis_mathclap_24Jun2003.pdf) of items 1 thru 5. Except for `\smash`'s optional argument. It's described in the [AMS User's Guide](http://texdoc.net/texmf-dist/doc/latex/amsmath/amsldoc.pdf).
Item 6 also comes from the AMS User's Guide.
* Fix test spec
* Exploit makeVList for smash
* update smash and phantom screenshots for chrome
* Pick up review comments
* Change test from \llap to \mathlap
\llap is fundamentally a text-mode function. We should not expect it to work correctly when given math-mode arguments. So test \mathllap instead.
* Correct \llap macro
A correction. The previous macro returned an error if given an argument with math-mode content, such as x^2.
The corrected macro will not return an error. It will instead return well rendered math, but letters are in `\mathrm` font.
* update \llap, \rlap, \clap macros to use \textrm
* update Lap screenshots
