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

static/katex.less

@@ -350,7 +350,8 @@
     }
 
     .llap,
-    .rlap {
+    .rlap,
+    .clap {
         width: 0;
         position: relative;
 
@@ -367,10 +368,16 @@
         right: 0;
     }
 
-    .rlap > .inner {
+    .rlap > .inner,
+    .clap > .inner {
         left: 0;
     }
 
+    .clap > .inner  > span {
+        margin-left: -50%;
+        margin-right: 50%;
+    }
+
     .katex-logo {
         .a {
             font-size: 0.75em;