faster contentsplitbrackets; degenerate case check when evaluating strings

contentsplitbrackets uses regexps now.
Evaluating strings did a call to contentsubvars which was unnecessary the vast majority of the time. Now check if the string contains a `{` character before doing contentsubvars, to save time.
\var and \simplify implemented as MathJax commands

Should be a bit faster but, more importantly, I don't have to pretend to parse LaTeX! Should be much less buggy this way.
As a side benefit, TeX spread across several lines (with <br/> tags) works now.