Don't leak AllowStd type outside the crate

It's not needed as public API and an implementation detail.

src/compat.rs

@@ -14,7 +14,7 @@ pub(crate) enum ContextWaker {
 }
 
 #[derive(Debug)]
-pub struct AllowStd<S> {
+pub(crate) struct AllowStd<S> {
     inner: S,
     // We have the problem that external read operations (i.e. the Stream impl)
     // can trigger both read (AsyncRead) and write (AsyncWrite) operations on

src/lib.rs

@@ -254,7 +254,6 @@ impl<S> WebSocketStream<S> {
 impl<T> Stream for WebSocketStream<T>
 where
     T: AsyncRead + AsyncWrite + Unpin,
-    AllowStd<T>: Read + Write,
 {
     type Item = Result<Message, WsError>;
 
@@ -278,7 +277,6 @@ where
 impl<T> Sink<Message> for WebSocketStream<T>
 where
     T: AsyncRead + AsyncWrite + Unpin,
-    AllowStd<T>: Read + Write,
 {
     type Error = WsError;