Implement Rule Hash for Selector Matching.

* Using Rule Hash, we can implement the parallel optimizations for CSS Selector Matching from [Fast and parallel web page layout (2010)](http://www.eecs.berkeley.edu/~lmeyerov/projects/pbrowser/pubfiles/paper.pdf)
* Add index fields in Rule for parent StyleRule and parent Stylesheet to break ties while cascading.
  Sorting by (specificity, stylesheet index, rule index) removes the need to use stable sorts.