Further TLObject generator clean-up

Split everything into several functions, reused some more
common code (like accessing the "real" arguments instead
constantly filtering) and more, like using classmethods
instead staticmethods and then hardcoding the class name.

telethon/tl/tlobject.py

@@ -189,6 +189,6 @@ class TLObject:
     def __bytes__(self):
         return b''
 
-    @staticmethod
-    def from_reader(reader):
+    @classmethod
+    def from_reader(cls, reader):
         return TLObject()