chore: Revert "use #[non_exhaustive] instead of private unit field" (#3323)

This reverts commit 575938d457.

tokio-util/src/codec/bytes_codec.rs

@@ -42,13 +42,12 @@ use std::io;
 /// ```
 ///
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Default)]
-#[non_exhaustive]
-pub struct BytesCodec;
+pub struct BytesCodec(());
 
 impl BytesCodec {
     /// Creates a new `BytesCodec` for shipping around raw bytes.
     pub fn new() -> BytesCodec {
-        BytesCodec
+        BytesCodec(())
     }
 }