scons/swig: refactor some of the scons/SWIG code

- Move the random bits of SWIG code generation out of src/SConscript
  file and into methods on the objects being wrapped.
- Cleaned up some variable naming and added some comments to make
  the process a little clearer.
- Did a little generated file/module renaming:
   - vptype_Foo now Foo_vector
   - init_Foo is now Foo_init
  This makes it easier to see all the Foo-related files in a
  sorted directory listing.
- Made cxx_predecls and swig_predecls normal SimObject classmethods.
- Got rid of swig_objdecls hook, even though this breaks the System
  objects get/setMemoryMode method exports.  Will be fixing this in
  a future changeset.

src/sim/System.py

@@ -40,6 +40,8 @@ class MemoryMode(Enum): vals = ['invalid', 'atomic', 'timing']
 class System(SimObject):
     type = 'System'
 
+    # This method is temporarily obsolete.  Its functionality will be
+    # restored in a future changeset.
     @classmethod
     def swig_objdecls(cls, code):
         code('%include "python/swig/system.i"')