Add GetExpressionIdentifiers to the Expression Engine

[gregboone]

Add a new method 'FdoExpressionEngine::GetExpressionIdentifiers' to the Expression Engine to allow a caller to retrieve the list of identifiers specified in an expression statement.

[gregboone]

This will be a static method:

    /// \brief
    /// Returns the list of class identifiers referenced by an expression
    /// 
    /// \param functions
    /// Input a list of functions to be used when processing function type expressions
    ///
    /// \param originalClassDef
    /// Input the class definition applicable to the expression being evaluated
    ///
    /// \param expression
    /// Input the expression to be evaluated
    ///
    /// \param identifiers
    /// Output the IdentifierCollection that will be populated with the list of class 
    /// identifiers referenced by the expression
    ///
    /// \return
    /// Returns nothing
    /// 
    EXPRESSIONENGINE_API static void GetExpressionIdentifiers(FdoFunctionDefinitionCollection *functionDefinitions, 
                                                              FdoClassDefinition* originalClassDef, 
                                                              FdoExpression *expression,
                                                              FdoIdentifierCollection* identifiers);

    /// \brief
    /// Returns the list of class identifiers referenced by an expression
    ///
    /// \param originalClassDef
    /// Input the class definition applicable to the expression being evaluated
    ///
    /// \param expression
    /// Input the expression to be evaluated
    ///
    /// \param identifiers
    /// Output the IdentifierCollection that will be populated with the list of class 
    /// identifiers referenced by the expression
    ///
    /// \return
    /// Returns nothing
    /// 
    EXPRESSIONENGINE_API static void GetExpressionIdentifiers(FdoClassDefinition* originalClassDef, 
                                                              FdoExpression *expression,
                                                              FdoIdentifierCollection* identifiers);