XStream should provide helpful exception if serializing itself

Currently XStream cannot serialize itself as it's internal state is changing whilst serialization occurs (see XSTR-50).

When this happens a ConcurrentModificationException is thrown but it is not clear why to users. I would like XStream to detect if itself is being serialized (or any of its internal classes such as mappers) and throw an easy to understand exception (such as XStreamCannotSerializeItselfException).

Note, it should be possible for XStream to serialize other instances of other XStreams.