Advance index when reading multiple bytes as well as one at a time

implementations/rust/src/value/decoder.rs

@@ -122,7 +122,10 @@ impl<'a, 'b, R: Read, N: NestedValue> Decoder<'a, 'b, R, N> {
         }
         let mut bs = vec![0; n];
         match self.read.read_exact(&mut bs) {
-            Ok(()) => Ok(bs),
+            Ok(()) => {
+                self.index += n;
+                Ok(bs)
+            }
             Err(e) =>
                 if e.kind() == std::io::ErrorKind::UnexpectedEof {
                     Err(Error::Eof)