Replace TLObject.on_send with the new .to_bytes()

This also replaces some int.to_bytes() calls with a faster
struct.pack (up to x4 faster). This approach is also around
x3 faster than creating a BinaryWriter just to serialize a
TLObject as bytes.

telethon/extensions/binary_writer.py

@@ -110,7 +110,7 @@ class BinaryWriter:
 
     def tgwrite_object(self, tlobject):
         """Writes a Telegram object"""
-        tlobject.on_send(self)
+        self.write(tlobject.to_bytes())
 
     def tgwrite_vector(self, vector):
         """Writes a vector of Telegram objects"""