[DAG-copy] Implement DAG-aware visitor

After certain replacements, e.g., formula substitution, the AST becomes a directed acyclic graph.
Most often it is not required to visit a node of this DAG multiply times.
The new class DAGVisitor keeps track of the nodes already seen and visits each node only once.

The class DAGVisitor is inserted between ASTVisitor and ASTTraverse and ASTTraverseModify
without changing the behavior of their subclasses.

Author: Steffen Märcker