Initial skeleton of new Broxygen infrastructure.

Doesn't generate any docs, but it's hooked in to all places needed to
gather the necessary stuff w/ significantly less coupling than before.

The gathering now always occurs unconditionally to make documentation
available at runtime and a command line switch (-X) only toggles whether
to output docs to disk (reST format).

Should also improve the treatment of type name aliasing which wasn't a
big problem in practice before, but I think it's more correct now:
there's now a distinct BroType for each alias, but extensible types
(record/enum) will automatically update the types for aliases on redef.

Other misc refactoring of note:

    - Removed a redundant/unused way of declaring event types.

    - Changed type serialization format/process to preserve type name
      information and remove compatibility code (since broccoli will
      have be updated anyway).

src/Serializer.h

@@ -125,7 +125,7 @@ protected:
 
 	// This will be increased whenever there is an incompatible change
 	// in the data format.
-	static const uint32 DATA_FORMAT_VERSION = 23;
+	static const uint32 DATA_FORMAT_VERSION = 24;
 
 	ChunkedIO* io;