Class Walker

An iterator for walking a Node tree.

This is depth-first, children are visited before the parent.
The Walker object can be initialized with any node, and
returns the next node on the descent with each next() call.
'kids_func' is an optional function that will be called to
get the children of a node instead of calling 'children'.
'cycle_func' is an optional function that will be called
when a cycle is detected.

This class does not get caught in node cycles caused, for example,
by C header file include loops.

Return the next node for this walk of the tree.

This function is intentionally iterative, not recursive,
to sidestep any issues of stack size limitations.