Add \smash, laps, spaces, and phantoms (#833)

* 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

test/screenshotter/ss_data.yaml

@@ -107,7 +107,7 @@ KaTeX: \KaTeX
 Kern:
     tex: \frac{a\kern{1em}b}{c}a\kern{1em}b\kern{1ex}c\kern{-0.25em}d
     nolatex: LaTeX fails to typeset this, “Missing number, treated as zero.”
-Lap: ab\llap{f}cd\rlap{g}h
+Lap: ab\mathllap{f}cd\mathrlap{g}hij\mathclap{k}lm \; ab\llap{f}cd\rlap{g}hij\clap{k}lm
 LargeRuleNumerator: \frac{\textcolor{blue}{\rule{1em}{2em}}}{x}
 LeftRight: \left( x^2 \right) \left\{ x^{x^{x^{x^x}}} \right.
 LeftRightListStyling: a+\left(x+y\right)-x
@@ -178,7 +178,7 @@ OverUnderset: |
     a+b+c+d\overset{b+c=0}\longrightarrow a+d\\
     \overset { x = y } { \sqrt { a b } }
     \end{array}
-Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x}
+Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x} \left(\vphantom{\int_t} zzz \right) \left( X \hphantom{\frac{\frac X X}{X}} \right)
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2
 RelativeUnits: |
@@ -190,7 +190,7 @@ RelativeUnits: |
         \rule{1em}{1em}^{\rule{1em}{1em}}\rule{18mu}{18mu}^{\rule{18mu}{18mu}} &
         {\footnotesize\rule{1em}{1em}^{\rule{1em}{1em}}\rule{18mu}{18mu}^{\rule{18mu}{18mu}}}
     \end{array}
-RlapBug: \frac{\rlap{x}}{2}
+RlapBug: \frac{\mathrlap{x}}{2}
 Rule: \rule{1em}{0.5em}\rule{1ex}{2ex}\rule{1em}{1ex}\rule{1em}{0.431ex}
 SizingBaseline:
     tex: '{\tiny a+b}a+b{\Huge a+b}'
@@ -198,6 +198,7 @@ SizingBaseline:
     post: M
 Sizing: |
     {\Huge x}{\LARGE y}{\normalsize z}{\scriptsize w}
+Smash: \left( X^{\smash 2} \right) \sqrt{\smash[b]{y}}
 Spacing: ^3+[-1][1-1]1=1(=1)\lvert a\rvert~b
 Sqrt: |
     \sqrt{\sqrt{\sqrt{x}}}_{\sqrt{\sqrt{x}}}^{\sqrt{\sqrt{\sqrt{x}}}