Create RpcResult class and generalise core special cases

This results in a cleaner MTProtoSender, which now can always
read a TLObject with a guaranteed item, if the message is OK.

telethon/tl/core/messagecontainer.py

@@ -0,0 +1,53 @@
+import struct
+
+from ..tlobject import TLObject
+from .tlmessage import TLMessage
+
+
+class MessageContainer(TLObject):
+    CONSTRUCTOR_ID = 0x73f1f8dc
+
+    def __init__(self, messages):
+        super().__init__()
+        self.content_related = False
+        self.messages = messages
+
+    def to_dict(self, recursive=True):
+        return {
+            'content_related': self.content_related,
+            'messages':
+                ([] if self.messages is None else [
+                    None if x is None else x.to_dict() for x in self.messages
+                ]) if recursive else self.messages,
+        }
+
+    def __bytes__(self):
+        return struct.pack(
+            '<Ii', MessageContainer.CONSTRUCTOR_ID, len(self.messages)
+        ) + b''.join(bytes(m) for m in self.messages)
+
+    @staticmethod
+    def iter_read(reader):
+        reader.read_int(signed=False)  # code
+        size = reader.read_int()
+        for _ in range(size):
+            inner_msg_id = reader.read_long()
+            inner_sequence = reader.read_int()
+            inner_length = reader.read_int()
+            yield TLMessage(inner_msg_id, inner_sequence,
+                            body=reader.read(inner_length))
+
+    def __str__(self):
+        return TLObject.pretty_format(self)
+
+    def stringify(self):
+        return TLObject.pretty_format(self, indent=0)
+
+    @classmethod
+    def from_reader(cls, reader):
+        # This assumes that .read_* calls are done in the order they appear
+        return MessageContainer([TLMessage(
+            msg_id=reader.read_long(),
+            seq_no=reader.read_int(),
+            body=reader.read(reader.read_int())
+        ) for _ in range(reader.read_int())])