Move common C extension module into a different C file. (#35)

* Move common C extension module into a different C file so that the actual definitions can be more easily included in other C extensions.

* Rename common_extension_module -> common_module.

lib/python/common_extension.h

@@ -24,6 +24,8 @@ typedef struct {
 
 extern PyTypeObject PyObjectIDType;
 
+extern PyTypeObject PyTaskType;
+
 int PyObjectToUniqueID(PyObject *object, object_id *objectid);
 
 PyObject *PyObjectID_make(object_id object_id);