z3_solver: unary operator parse support

This solve the following issue reported by wexlermi

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Crest-Z3 does not allow for the unary minus operator
This fork of Crest does not allow for the unary minus operator. For instance, the following constraint will crash Crest:

if (x*x*x == -8) {
printf("here");
}

Subtraction seems to be allowed, but any time the unary minus operator appears, Crest crashes.
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