Add additional benchmark documents

The document and node structures differ significantly in these new
documents compared to the existing benchmarks. This will help uncover
new bottlenecks in the system.
Correct IE detection code for containsNode workaround
Fix PositionIterator bug when starting near rejected text nodes

TODO
- Unit tests
Avoid more slow calls in StepsTranslator

Add more complex equality & lookup logic that attempts to avoid calls to
unfilteredDomOffset.

TODO
- Move & rename to PositionMemento
- Potentially expose getter on PositionIterator
- Unit tests
Improve performance by moving node filters out of the TextPositionFilter

TODO
- Unit test
- Redesign to fit in the correct area
Put off accessing the walker's current node as long as possible

Fetching the currentNode property on a tree walker is surprisingly slow.
Putting this off as long as possible improves the overall iteration
speed significantly.