* \color affects following \right
Fix#1844 by giving `leftright` nodes a `rightColor` attribute for how
to color the right bracket. Now `\color` sets the macro
`\current@color` in the current environment (in particular, resetting
after `\right`), just like `color.sty` does in LaTeX. This is used to
specially pass the current color into any following `\right` and then
into the `leftright` parse node.
* Add test
* Put each array cell in its own group/namespace
* Improve cell group isolation, add test and TODO
* Improve comments
