Rewrite spacing commands as macros (#1156)

* Rewrite spacing commands as macros

Fix #1130 by defining `\!`, `\negthinspace`, `\,`, `\thinspace`, `\:`,
`\medspace`, `\;`, `\thickspace`, `\negmedspace`, `\negthickspace`,
`\enspace`, `\enskip`, `\qquad`, `\quad` as macros.

Fix #1036 by defining a new `SpaceNode` in mathMLTree that recognizes
all special space amounts from
https://www.w3.org/TR/2000/WD-MathML2-20000328/chapter6.html

* Fix <mspace> rendering and add a test to catch it

* Update screenshots

* Wrap special space characters in <mtext>

* Update screenshots

* Fix MathML escaping behavior

* Fix flow typos

* Fix Unicode-chrome

* Reimplement mod operators in terms of macros

* Rerun ModScript

* Remove excess table entries (now already macros)

* Fix bmod

src/katex.less

@@ -244,54 +244,6 @@
 
     .mspace {
         display: inline-block;
-
-        &.negativethinspace {
-            margin-left: -@thinspace;
-        }
-
-        &.muspace {
-            width: @muspace;
-        }
-
-        &.thinspace {
-            width: @thinspace;
-        }
-
-        &.negativemediumspace {
-            margin-left: -@mediumspace;
-        }
-
-        &.mediumspace {
-            width: @mediumspace;
-        }
-
-        &.thickspace {
-            width: @thickspace;
-        }
-
-        &.sixmuspace {
-            width: 0.333333em;
-        }
-
-        &.eightmuspace {
-            width: 0.444444em;
-        }
-
-        &.enspace {
-            width: 0.5em;
-        }
-
-        &.twelvemuspace {
-            width: 0.666667em;
-        }
-
-        &.quad {
-            width: 1em;
-        }
-
-        &.qquad {
-            width: 2em;
-        }
     }
 
     .llap,