message: Fix unused field warnings
  We only need to store an encrypted chunk across multiple calls to
  AsyncWrite::poll_write; when writing synchronously, we write the entire
  encrypted chunk inline.
diff: @@ -65,6 +65,7 @@ impl Nonce {
  }
  }
  +#[cfg(feature = "async")]
  struct EncryptedChunk {
  bytes: Vec<u8>,
  offset: usize,
  @@ -102,6 +103,7 @@ impl Stream {
  stream: Self::new(key),
  inner,
  chunk: Vec::with_capacity(CHUNK_SIZE),
  + #[cfg(feature = "async")]
  encrypted_chunk: None,
  }
  }
  @@ -210,6 +212,7 @@ pub struct StreamWriter<W> {
  #[pin]
  inner: W,
  chunk: Vec<u8>,
  + #[cfg(feature = "async")]
  encrypted_chunk: Option<EncryptedChunk>,
  }
  