Derive Debug for io::Chain instead of manually implementing it.

The manual implementation has the same bounds, so I don't think there's
any reason for a manual implementation. The names used in the derive
implementation are even nicer (`first`/`second`) than the manual
implementation (`t`/`u`), and include the `done_first` field too.

library/std/src/io/mod.rs

@@ -2114,6 +2114,7 @@ fn lines(self) -> Lines<Self>
 ///
 /// [`chain`]: Read::chain
 #[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
 pub struct Chain<T, U> {
     first: T,
     second: U,
@@ -2195,13 +2196,6 @@ pub fn get_mut(&mut self) -> (&mut T, &mut U) {
     }
 }
 
-#[stable(feature = "std_debug", since = "1.16.0")]
-impl<T: fmt::Debug, U: fmt::Debug> fmt::Debug for Chain<T, U> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("Chain").field("t", &self.first).field("u", &self.second).finish()
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Read, U: Read> Read for Chain<T, U> {
     fn read(&mut self, buf: &mut [u8]) -> Result<usize> {