RCR 323: Object Graph Traversal in Standard Lib

Make a standard library function available that starts with a given object and traverses all objects reachable from there properly dealing with recursion.

It comes up frequently on ruby-talk that people want to traverse object graphs to do something with each instance. Although there are certainly specialized traversals needed a great deal of them just visits every instance and does something with it.

Add a method (for example Kernel.traverse(obj,&b)) which receives a starting object and a block. It traverses through the object graph and yields every instance found at most once to the block. Proper handling of recursive structures included of course. An alternative interface might be Kernel.traverse(*objs,&b).

It's more efficient to have this in C code and only once so everybody can use it. Also it seems a fairly general thing to do. Module pp might also benefit.

The code should be there already (in Marshal), there's just another interface to it needed.