Updates the following

• Adds roxmltree parsing
• Updates ast to use arenas
• Implementations now specific to parsing, using a common ast representation
• xml namspaces now handled in parsing instead of serializing, which means a lot of blocked/broken stuff is/can be resolved
• Uses XmlWriter for more flexible serialization

Adds the following regressions

• PI expressions aren't serialized accurately
• 28 more broken w3c tests (default preset)
• 72 more broken w3c tests (none preset)
• 209 more broken oxygen icons (default preset)
• 224 more broken oxygen icons (none preset)

Side effects of changes

• Some characters are no longer serialized as escape sequences