Specify traversal and overhaul ongoing navigation tracking

This commit introduces a much more robust framework for tracking ongoing navigations. This fixes #130, for example, by appropriately keeping the signal around so that we can signal abort when necessary. It also ensures every same-document navigation ends in either navigationsuccess or navigationerror, with a few unified spec paths.

With this framework in hand, we can specify goTo()/back()/forward().

Author: domenic