Create a default module to use as a sentinel value

This looks better in the documentation than utils.Default,
cleans the utils with specific stuff like this, and users
may use it more easily.

telethon/helpers.py

@@ -1,6 +1,5 @@
 """Various helpers not related to the Telegram API itself"""
 import asyncio
-import collections
 import os
 import struct
 from hashlib import sha1, sha256