io: Rename ReadBuf methods (#2945)

This changes `ReadBuf::add_filled` to `ReadBuf::advance` and
`ReadBuf::append` to `ReadBuf::put_slice`. This is just a
mechanical change.

Closes #2769

tokio-util/src/io/stream_reader.rs

@@ -114,7 +114,7 @@ where
             Poll::Pending => return Poll::Pending,
         };
         let len = std::cmp::min(inner_buf.len(), buf.remaining());
-        buf.append(&inner_buf[..len]);
+        buf.put_slice(&inner_buf[..len]);
 
         self.consume(len);
         Poll::Ready(Ok(()))