Fixed Sugarcane rewind menu bug

* This should fix the bug where setter links' variable changes don't
work when backtracking via Sugarcane's Rewind menu.
* This also hopefully fixes the bug where, when using HTML5 history, the
deserialised history array's contents lacked the Passage prototyping.
* Added Passage.prototype.toJSON() to fix the above, which simply
reduces passages to their ID number. This also means that the serialised
game state used for HTML5 history is now a tad smaller.
* Also altered the names of a few engine variables here and there.