Pre-pack outgoing TLMessage

This has several benefits:
- The message can be resent without re-calling bytes(),
  which for some requests may be expensive.
- Converting requests to bytes early lets us detect
  errors early, such as OverflowError on bad requests.
- Containers can't exceed 1044456 bytes so knowing their
  length is important. This can now be done in O(1).

But also several drawbacks:
- If the object is modified the bytes won't reflect this.
  This isn't an issue because it's only done for in msgs.
- Incoming messages can no longer be reconverted into
  bytes but this was never needed anyway.

telethon/network/mtprotosender.py

@@ -507,7 +507,6 @@ class MTProtoSender:
                       rpc_result.req_msg_id)
 
         if rpc_result.error:
-            # TODO Report errors if possible/enabled
             error = rpc_message_to_error(rpc_result.error)
             self._send_queue.put_nowait(self.state.create_message(
                 MsgsAck([message.msg_id])
@@ -517,10 +516,13 @@ class MTProtoSender:
                 message.future.set_exception(error)
             return
         elif message:
+            # TODO Would be nice to avoid accessing a per-obj read_result
+            # Instead have a variable that indicated how the result should
+            # be read (an enum) and dispatch to read the result, mostly
+            # always it's just a normal TLObject.
             with BinaryReader(rpc_result.body) as reader:
                 result = message.obj.read_result(reader)
 
-            # TODO Process entities
             if not message.future.cancelled():
                 message.future.set_result(result)
             return