Move varint and delim read/write to toplevel

To `libp2p.utils`.

libp2p/stream_muxer/abc.py

@@ -17,7 +17,6 @@ class IMuxedConn(ABC):
     reference: https://github.com/libp2p/go-stream-muxer/blob/master/muxer.go
     """
 
-    initiator: bool
     peer_id: ID
 
     @abstractmethod
@@ -35,6 +34,11 @@ class IMuxedConn(ABC):
         :param peer_id: peer_id of peer the connection is to
         """
 
+    @property
+    @abstractmethod
+    def initiator(self) -> bool:
+        pass
+
     @abstractmethod
     def close(self) -> None:
         """