Add requested macros and environments now that we can support them.

[dpvc]

This PR adds a number of macros and environments that have been requested in the past, now that we have fonts that contain the needed characters. These include \AA, \iddots, \oiint and \oiiint, plus I added \dddot and \ddddot accents, and the missing \ointop. The TEXCLASS.OP has been removed from the big operators, since those will be inherited from the operator dictionary, so no need to waste the space on redundant values. The unicode value for \backslash has been changed to just \.

The new environments are darray for an array in display mode, and math and displaymath. Since \begin{xyz}...\end{xyz} are parsed as math-mode delimiters, and these default to display mode, the math environment requires the ability to change the MathItem's display property to false for this environment. That required adding the active MathItem to the ParseOptions instance so that the math environment could change it. In turn, that means the TeX input jax can't cache the display value, since it can change during the parsing process.

Resolves:

• Implement TeX commands for Contour Integral MathJax#566
• Angstrom symbol (\AA) does not render in MathJax MathJax#795
• math, displaymath environments MathJax#996
• about mathdots packages \iddots MathJax#1269
• Incorrect Unicode Constants - Wikimedia Issue T105315 MathJax#1937
• Could you give me a Surface Integral Character MathJax#2838
• Support darray Unknown environment 'darray' MathJax#2878

[zorkow]

Some minor things.