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/rpcresult.py

@@ -0,0 +1,23 @@
+from .gzippacked import GzipPacked
+from ..types import RpcError
+
+
+class RpcResult:
+    CONSTRUCTOR_ID = 0xf35c6d01
+
+    def __init__(self, req_msg_id, body, error):
+        self.req_msg_id = req_msg_id
+        self.body = body
+        self.error = error
+
+    @classmethod
+    def from_reader(cls, reader):
+        msg_id = reader.read_long()
+        inner_code = reader.read_int(signed=False)
+        if inner_code == RpcError.CONSTRUCTOR_ID:
+            return RpcResult(msg_id, None, RpcError.from_reader(reader))
+        if inner_code == GzipPacked.CONSTRUCTOR_ID:
+            return RpcResult(msg_id, GzipPacked.from_reader(reader).data, None)
+
+        reader.seek(-4)
+        return RpcResult(msg_id, reader.read(), None)