wandering around XPathDocument2

After making XmlDocumentNavigator editable, today I was trying to create another XPathDocument based on another tree model like XOM... that will be editable in a few days (on the same way as shown in XmlDocumentEditableNavigator), but I hoped too much for today's XML toy. In fact the basic tree model is created nearly a week ago, so today I just added Load() and XPathNavigator implementation (though I had to modify the large part of the tree model). Right now it just implements IXPathNavigable (it won't be difficult though), and has some limitation (e.g. MoveToId() won't work right now).

In fact, at first I intended to create more useful document API like XOM for new XPathDocument, but I found that new XPathDocument had better be more simple implementation that is not kind for "users" (such like omitting parameter checks). Thus, the tree model now became so different from original XOM way.

Like MS one, this XPathNavigator is "a bit" faster than XmlDocumentNavigator, but it is not faster than our DTMXPathNavigator. I just tried only those navigation API used in XPathNavigatorReader, so on reverse axes it will be much faster than XmlDocumentNavigator.

This code is just two days' hack, so it might have many bugs. Well, yesterday's XPathNavigatorReader was so buggy and I've fixed many bugs in cvs.