implement non-const iteration on Nodes

We provide an API supporting range-based for loops and legacy iteration over an
AST or a subtree rooted at a particular node. The purpose of this is to abstract
patterns like indexing and validation into a common implementation. This needs a
const counterpart, but then it should be ready to near-entirely replace the
functionality of indexing, validation, and friends. This should then be a
suitable base to build Node::all, Node::any, Node::none, etc on top of.

Github: related to #152 "traversal abort"

librumur/include/rumur/Node.h

@@ -4,6 +4,7 @@
 #include <cstddef>
 #include <iostream>
 #include "location.hh"
+#include <vector>
 
 namespace rumur {
 
@@ -28,6 +29,35 @@ struct Node {
    */
   virtual void validate() const { }
 
+  // Iteration-supporting infrastructure. You do not need to pay much attention
+  // to the following details, but their purpose is to allow you to use C++11
+  // range-based for loops on AST nodes:
+  //
+  //   for (Node *n : myAST) {
+  //     ...
+  //   }
+  //
+  // The traversal order is not guaranteed, so do not use this API if you
+  // specifically require pre-order or post-order.
+
+  class Iterator {
+
+   private:
+    std::vector<Node*> remaining;
+
+   public:
+    explicit Iterator();
+    explicit Iterator(Node &base);
+    Iterator &operator++();
+    Iterator operator++(int);
+    Node *operator*();
+    bool operator==(const Iterator &other) const;
+    bool operator!=(const Iterator &other) const;
+  };
+
+  Iterator begin();
+  Iterator end();
+
 };
 
 }