This CL introduces the general visitor functionality that will replace
the Left, SetLeft, Right, SetRight, etc methods in the Node interface.
For now, the CL defines the functionality in terms of those methods,
but eventually the Nodes themselves will implement DoChildren
and EditChildren and be relieved of implementing Left, SetLeft, and so on.
The CL also updates Inspect (which moved to visit.go) and DeepCopy
to use the new functionality.
The Find helper is not used in this CL but will be used in a future one.