style: [patch 3/22] reduce include dependencies in some headers

Used cppclean to help identify useless includes and removed them. This
involved erroneously included headers, but also cases where forward
declarations could have been used rather than a full include.

src/sim/serialize.hh

@@ -59,14 +59,12 @@
 #include <vector>
 
 #include "base/bitunion.hh"
-#include "base/types.hh"
 
+class CheckpointIn;
 class IniFile;
 class Serializable;
-class CheckpointIn;
 class SimObject;
 class SimObjectResolver;
-class EventQueue;
 
 typedef std::ostream CheckpointOut;