codec: deprecate encoder's encode/flush and decoder's decode

src/codec/decoder/audio.rs

@@ -12,6 +12,11 @@ use {packet, AudioService, ChannelLayout, Error};
 pub struct Audio(pub Opened);
 
 impl Audio {
+    #[deprecated(
+        since = "4.4.0",
+        note = "Underlying API avcodec_decode_audio4 has been deprecated since FFmpeg 3.1; \
+        consider switching to send_packet() and receive_frame()"
+    )]
     pub fn decode<P: packet::Ref>(
         &mut self,
         packet: &P,