Began reworking the network code

This commit devides SerialPacket into a series of different structures,
all decended from a common base class.

Using a union was not a good idea.

common/gameplay/sanity_check.cpp

@@ -31,7 +31,4 @@
  * Since most/all of the files in this directory are header files, I've created
  * this source file as a "sanity check", to ensure that the above header files
  * are written correctly via make.
- * 
- * Oddly enough, I'm pretty sure this is the first directory compiled in a
- * clean build.
 */