TrueFormula and FalseFormula

[ibnyusuf]

In Formula.hpp we define classes TrueFormula and FalseFormula:

    class TrueFormula : public Formula
    {
        friend class Formulas;
        
    public:
        TrueFormula(std::string label = "") : Formula(label) {}
        
        Type type() const override { return Formula::Type::True; }
        std::string toSMTLIB(unsigned indentation = 0) const override;
        std::string prettyString(unsigned indentation = 0) const override;
    };
    
    class FalseFormula : public Formula
    {
        friend class Formulas;
        
    public:
        FalseFormula(std::string label = "") : Formula(label) {}
        
        Type type() const override { return Formula::Type::False; }
        std::string toSMTLIB(unsigned indentation = 0) const override;
        std::string prettyString(unsigned indentation = 0) const override;
    };

But then never use these elsewhere. Instead the function boolTrue() declared in Theory.hpp creates a predicate formula with name true. We should either remove these two classes or update Theory.hpp to make use of them.