fix navigation, update TOC, fix a few xrefs, stub out serializing chapter

diveintopython3.org

@@ -2,7 +2,7 @@
 * Your First Python Program
 ** TODO mention why from module import * is only allowed at module level
 * Native Datatypes
-* TODO Comprehensions
+* DONE Comprehensions
 ** List comprehensions
 ** Set comprehensions
 ** Dictionary comprehensions
@@ -45,7 +45,7 @@
 * TODO 2nd draft Porting Code to Python 3 with 2to3
 * TODO 2nd draft Special Method Names
 * Bits to add somewhere
-** TODO section on tuples
+** DONE section on tuples
 ** TODO section on dictionary views
 several dictionary methods return them
 they're dynamic