Hide accidentally public constructor for gio::IOStreamAsyncReadWrite

A similar type should be provided by the gio crate or an extension crate
like https://github.com/sdroege/gio-futures

src/gio.rs

@@ -71,7 +71,7 @@ pub struct IOStreamAsyncReadWrite<T: IsA<gio::IOStream>> {
 
 impl<T: IsA<gio::IOStream>> IOStreamAsyncReadWrite<T> {
     /// Create a new `gio::IOStream` adapter
-    pub fn new(stream: T) -> Result<IOStreamAsyncReadWrite<T>, T> {
+    fn new(stream: T) -> Result<IOStreamAsyncReadWrite<T>, T> {
         let write = stream
             .get_output_stream()
             .and_then(|s| s.dynamic_cast::<gio::PollableOutputStream>().ok())