substitute incorrectly handles negative numbers when reduceConstants is set to 0

[dlglin]

See the attached MWE. If reduceConstants is set to 0, when a negative number is substituted into a formula, the parentheses are removed in the LaTeX string.

e.g.
Context() -> flags -> set(reduceConstants => 0);
$f = Formula("x^2");
$g = $f->substitute(x=>-3);

The value of $g is 9, but the LaTeX string for $g is "-3^2", which causes two issues:

1. $g is displayed incorrectly (without the parentheses) when shown on screen.
2. If $g is used in a subsequent calculation, then the LaTeX string is used, which leads to incorrect values. e.g. $g-1 evaluates to -10

reduceConstantsIssue.pg.txt

[pstaabp]

#659 now fixes this.