add closed() procedure to check if transport is closed.

asyncdispatch2/transports/datagram.nim

@@ -665,3 +665,7 @@ proc getMessage*(transp: DatagramTransport): seq[byte] =
 proc getUserData*[T](transp: DatagramTransport): T {.inline.} =
   ## Obtain user data stored in ``transp`` object.
   result = cast[T](transp.udata)
+
+proc closed*(transp: DatagramTransport): bool {.inline.} =
+  ## Returns ``true`` if transport in closed state.
+  result = ({ReadClosed, WriteClosed} * transp.state != {})

asyncdispatch2/transports/stream.nim

@@ -1213,3 +1213,7 @@ proc close*(transp: StreamTransport) =
     transp.state.incl({WriteClosed, ReadClosed})
     transp.future.complete()
     GC_unref(transp)
+
+proc closed*(transp: StreamTransport): bool {.inline.} =
+  ## Returns ``true`` if transport in closed state.
+  result = ({ReadClosed, WriteClosed} * transp.state != {})