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-test/src/io.rs

@@ -212,7 +212,7 @@ impl Inner {
                 let n = cmp::min(dst.remaining(), data.len());
 
                 // Copy the data into the `dst` slice
-                dst.append(&data[..n]);
+                dst.put_slice(&data[..n]);
 
                 // Drain the data from the source
                 data.drain(..n);