With this PR, the only remaining usage of UnsafeNode is the transition stuff, which is servo-only and probably going to be rewritten over the course of stylo. The parallel traversal is now fully typechecked and safe. \o/
r? @emilio
Source-Repo: https://github.com/servo/servo
Source-Revision: b35ab98132539d4e6f1fcd4b3307eae40e387861