Avoid crashing with a StackOverflowException when iterating over the AllNodes property when it's infinitely recursive

[mlaily]

The implementations of AllNodes are delegated to an abstract internal IEnumerable<YamlNode> SafeAllNodes(RecursionLevel level) method that keeps track of the current recursion level and throws a MaximumRecursionLevelReachedException if the maximum recursion level is reached.

The maximum recursion level is currently set to 1000.

[mlaily]

I also implemented the same strategy in the YamlNode.ToString() methods, but instead of throwing an exception when the node is infinitely recursive, it breaks the infinite recursion by returning an error message as a value. (See the last three commits)

[aaubry]

I'm not sure about this. Maybe AllNodes should instead return each node only once?
What is the usefulness of this property if there is no guarantee that it will work for a given document?

[mlaily]

I don't know about the usefulness of AllNodes, but I'm pretty sure it's not supposed to crash the application if there is some infinite recursion :)

Returning each node only once would mean keeping a list of already returned nodes? This seems like a bad idea.

At least, with the code in this pull request, there is a catchable exception.

[aaubry]

Sorry, I did not have time to review this earlier. I agree that it is better if AllNodes and ToString do not crash. I'll leave the discussion about the usefulness of AllNodes for later. Thanks

[mlaily]

Thank you!