Advanced macro support and magic \dots (#794)

* Advanced macro support and magic \dots

* Fix \relax behavior

* Use \DOTSB in \iff, \implies, \impliedby

* Add multiple expansion test

* Implement some of @kevinbarash's comments

* More @kevinbarabash comments

* Token moved from merge

* Add type to defineMacro

* @flow

test/screenshotter/ss_data.yaml

@@ -77,6 +77,11 @@ DisplayMode:
 DisplayStyle: |
     {\displaystyle\sqrt{x}}{\sqrt{x}}
     {\displaystyle \frac12}{\frac12}{\displaystyle x^1_2}{x^1_2}
+Dots: |
+    \begin{array}{l}
+      \cdots;\dots+\dots\int\dots,\dots \\
+      \cdots{};\ldots+\ldots\int\ldots,\ldots
+    \end{array}
 Exponents: a^{a^a_a}_{a^a_a}
 ExtensibleArrows: |
     \begin{array}{l}