Implement environments, for arrays and matrices in particular

This commit introduces environments, and implements the parser
infrastructure to handle them, even including arguments after the
“\begin{name}” construct.  It also offers a way to turn array-like data
structures, i.e. delimited by “&” and “\\”, into nested arrays of groups.
Environments are essentially functions which call back to the parser to
parse their body.  It is their responsibility to stop at the next “\end”,
while the parser takes care of verifing that the names match between
“\begin” and “\end”.  The environment has to return a ParseResult, to
provide the position that goes with the resulting node.

One application of this is the “array” environment.  So far, it supports
column alignment, but no column separators, and no multi-column shorthands
using “*{…}”.  Building on the same infrastructure, there are “matrix”,
“pmatrix”, “bmatrix”, “vmatrix” and “Vmatrix” environments.  Internally
these are just “\left..\right” wrapped around an array with no margins at
its ends.  Spacing for arrays and matrices was derived from the LaTeX
sources, and comments indicate the appropriate references.

Now we have hard-wired breaks in parseExpression, to always break on “}”,
“\end”, “\right”, “&”, “\\” and “\cr”.  This means that these symbols are
never PART of an expression, at least not without some nesting.  They may
follow AFTER an expression, and the caller of parseExpression should be
expecting them.  The implicit groups for sizing or styling don't care what
ended the expression, which is all right for them.  We still have support
for breakOnToken, but now it is only used for “]” since that MAY be used to
terminate an optional argument, but otherwise it's an ordinary symbol.

test/screenshotter/ss_data.json

@@ -1,5 +1,6 @@
 {
     "Accents": "http://localhost:7936/test/screenshotter/test.html?m=\\vec{A}\\vec{x}\\vec x^2\\vec{x}_2^2\\vec{A}^2\\vec{xA}^2",
+    "Arrays": "http://localhost:7936/test/screenshotter/test.html?m=\\left(\\begin{array}{rlc}1%262%263\\\\1+1%262+1%263+1\\cr1\\over2%26\\scriptstyle 1/2%26\\frac12\\\\[1ex]\\begin{pmatrix}x\\\\y\\end{pmatrix}%260%26\\begin{vmatrix}a%26b\\\\c%26d\\end{vmatrix}\\end{array}\\right]",
     "Baseline": "http://localhost:7936/test/screenshotter/test.html?m=a+b-c\\cdot d/e",
     "BasicTest": "http://localhost:7936/test/screenshotter/test.html?m=a",
     "BinomTest": "http://localhost:7936/test/screenshotter/test.html?m=\\dbinom{a}{b}\\tbinom{a}{b}^{\\binom{a}{b}+17}",