Implement the DerefDescendantExpr #25
Description
Implementation of the DerefDescendantExpr currently is a copy of an old version of the DerefExpr, which drilled down into arrays and lookups the right operand for the objects in this array. Now the semantics for ->>foobar should be that it drills down into objects as well and to find values of objects, which have a foobar field in a predorder traversal.