Make sure negative numbers are handled as a negated number in Value items (#658)

[dpvc]

This PR fixes the problem where substituting a negative constant into a formula might not get the needed parentheses. It does so by providing a proper tree structure (negation together with a number) so that the parentheses are inserted when needed.

You can test using

Context()->variables->add(y => 'Real');
TEXT(Formula('x^y')->substitute(x => -3));

which should produce (-3)^y rather than -3^y, while

TEXT(Formula('x+y')->substitue(x => -3));

should produce -3+y (not (-3)+y).

Similarly,

TEXT(Formula('x^y')->substitue(x => 3));

should produce 3^y, as expected.

Resolves issue #658.

[pstaabp]

This looks good. I also checked:

TEXT(Formula('x+y')->substitute(x => -3));

which returns x+(-3). I assume that is intended and not x-3.

[dpvc]

@pstaabp, yes, this should produce x+(-3) just as Formula('x + -3') would. To get x-3 you need to use reduce.

[drgrice1]

This pull request seems to be causing a problem with the LimitedNumeric context. Opening a problem using this context causes the system to hang. A minimal working example is attached.
MWE.pg.txt

[drgrice1]

@dpvc: The problem seems to be that the class method for a Parser::Legacy::LimitedNumeric::UOP::minus object returns 'MINUS' instead of 'UOP'. As a result when the Parser::UOP new method is called, the isNeg method of lib/Parser/Item.pm returns false when it is called at line 33 of lib/Parser/UOP.pm. Thus the Parser::Item::Value new method is called, which calls the Parser::UOP new method (in the change introduce here) and starts it all over again, thus causing an infinite recursion.