util: add more get_{ref,mut,pin_mut} methods (#3364)

This commit adds:

- Framed::get_pin_mut
- FramedRead::get_pin_mut
- FramedWrite::get_pin_mut
- StreamReader::get_ref
- StreamReader::get_mut
- StreamReader::get_pin_mut
- StreamReader::into_inner

tokio-util/src/io/stream_reader.rs

@@ -93,6 +93,36 @@ where
     }
 }
 
+impl<S, B> StreamReader<S, B> {
+    /// Gets a reference to the underlying stream.
+    ///
+    /// It is inadvisable to directly read from the underlying stream.
+    pub fn get_ref(&self) -> &S {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying stream.
+    ///
+    /// It is inadvisable to directly read from the underlying stream.
+    pub fn get_mut(&mut self) -> &mut S {
+        &mut self.inner
+    }
+
+    /// Gets a pinned mutable reference to the underlying stream.
+    ///
+    /// It is inadvisable to directly read from the underlying stream.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut S> {
+        self.project().inner
+    }
+
+    /// Consumes this `BufWriter`, returning the underlying stream.
+    ///
+    /// Note that any leftover data in the internal buffer is lost.
+    pub fn into_inner(self) -> S {
+        self.inner
+    }
+}
+
 impl<S, B, E> AsyncRead for StreamReader<S, B>
 where
     S: Stream<Item = Result<B, E>>,