bolt-gen: keep single instance of type_obj per type

we rely, perhaps a bit hackily, on there only being one copy of
each type object floating about. using `deepcopy` on a Message
for message extensions destroys this paradigm, which breaks
things in the case where it's a later defined subtype that contains
variable-length members.

to fix this, we modify `__deepcopy__` on the Field class, such
that it preserves the reference to the original type_obj instance.

tools/generate-bolts.py

@@ -50,6 +50,15 @@ class Field(object):
         self.is_optional = optional
         self.field_comments = field_comments
 
+    def __deepcopy__(self, memo):
+        deepcopy_method = self.__deepcopy__
+        self.__deepcopy__ = None
+        field = copy.deepcopy(self, memo)
+        self.__deepcopy__ = deepcopy_method
+
+        field.type_obj = self.type_obj
+        return field
+
     def add_count(self, count):
         self.count = int(count)