Fixes #2 by ensuring that Xml2Doc is last to die.

[redvers]

In our previous iteration of this API we had Pony Class instances freeing C structs which were still referenced by Xml2Doc.

The documentation states that for our current parsing use-cases it is unsafe to automatically _final() free any of the XmlNodes as they're a part of our XmlDoc tree.

As such, we have threaded a tag reference through every dependent Pony instance so that it will be "impossible"™ for Xml2Doc._final() to execute the frees until all other dependent references are no longer reachable.

[redvers]

Hi @redvers,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

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