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/gzippacked.py

@@ -0,0 +1,42 @@
+import gzip
+import struct
+
+from .. import TLObject
+
+
+class GzipPacked(TLObject):
+    CONSTRUCTOR_ID = 0x3072cfa1
+
+    def __init__(self, data):
+        super().__init__()
+        self.data = data
+
+    @staticmethod
+    def gzip_if_smaller(request):
+        """Calls bytes(request), and based on a certain threshold,
+           optionally gzips the resulting data. If the gzipped data is
+           smaller than the original byte array, this is returned instead.
+
+           Note that this only applies to content related requests.
+        """
+        data = bytes(request)
+        # TODO This threshold could be configurable
+        if request.content_related and len(data) > 512:
+            gzipped = bytes(GzipPacked(data))
+            return gzipped if len(gzipped) < len(data) else data
+        else:
+            return data
+
+    def __bytes__(self):
+        # TODO Maybe compress level could be an option
+        return struct.pack('<I', GzipPacked.CONSTRUCTOR_ID) + \
+               TLObject.serialize_bytes(gzip.compress(self.data))
+
+    @staticmethod
+    def read(reader):
+        assert reader.read_int(signed=False) == GzipPacked.CONSTRUCTOR_ID
+        return gzip.decompress(reader.tgread_bytes())
+
+    @classmethod
+    def from_reader(cls, reader):
+        return GzipPacked(gzip.decompress(reader.tgread_bytes()))