Support \mathop, \mathrel, \mathbin, \mathpunct, etc. class commands.

These commands set their arguments in a given TeX math class. Use
the existing "op" type for \mathop (to support \limits); introduce
a new "mclass" type for the other classes.

Fixes #482. Tests borrowed from #485 (cbreeden).

test/screenshotter/ss_data.yaml

@@ -79,6 +79,8 @@ LeftRightStyleSizing: |
 LimitControls: |
     \displaystyle\int\limits_2^3 3x^2\,dx + \sum\nolimits^n_{i=1}i +
     \textstyle\int\limits_x^y z
+MathAtom: a\mathrel{\mathop{=}\limits^{\blue ?}}b
+MathAtom2: \mathop{\overline\mathrm{lim}}\limits_{x\to\infty}f(x)
 MathDefaultFonts: Ax2k\breve{a}\omega\Omega\imath+\KaTeX
 MathBb: \mathbb{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathBf: \mathbf{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}