Significant changes to Expression, and smaller changes to ConstantExpression and FieldPathExpression

This pull touches three files - Expression, ConstantExpression, and FieldPathExpression.  The former includes several changes. The changes are mostly trivial.  However, one of these requires team review.  Changes to ConstantExpression and FieldPathExpression included added a parse function to conform to the c++ code.  The parse method in both cases is trivial.