Implement fmt::Debug for all structures in libstd.

Part of https://github.com/rust-lang/rust/issues/31869.

Also turn on the `missing_debug_implementations` lint at the crate
level.

src/libstd/io/stdio.rs

@@ -282,6 +282,13 @@ impl Stdin {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Stdin {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stdin { .. }")
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Read for Stdin {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
@@ -314,6 +321,13 @@ impl<'a> BufRead for StdinLock<'a> {
     fn consume(&mut self, n: usize) { self.inner.consume(n) }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl<'a> fmt::Debug for StdinLock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("StdinLock { .. }")
+    }
+}
+
 /// A handle to the global standard output stream of the current process.
 ///
 /// Each handle shares a global buffer of data to be written to the standard
@@ -424,6 +438,13 @@ impl Stdout {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Stdout {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stdout { .. }")
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for Stdout {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
@@ -449,6 +470,13 @@ impl<'a> Write for StdoutLock<'a> {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl<'a> fmt::Debug for StdoutLock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("StdoutLock { .. }")
+    }
+}
+
 /// A handle to the standard error stream of a process.
 ///
 /// For more information, see the [`io::stderr`] method.
@@ -545,6 +573,13 @@ impl Stderr {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Stderr {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stderr { .. }")
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for Stderr {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
@@ -570,6 +605,13 @@ impl<'a> Write for StderrLock<'a> {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl<'a> fmt::Debug for StderrLock<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("StderrLock { .. }")
+    }
+}
+
 /// Resets the thread-local stderr handle to the specified writer
 ///
 /// This will replace the current thread's stderr handle, returning the old