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/mod.rs

@@ -1444,6 +1444,16 @@ pub struct Chain<T, U> {
     done_first: bool,
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+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> {
@@ -1485,6 +1495,7 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
 ///
 /// [`take()`]: trait.Read.html#method.take
 #[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
 pub struct Take<T> {
     inner: T,
     limit: u64,
@@ -1602,6 +1613,7 @@ fn read_one_byte(reader: &mut Read) -> Option<Result<u8>> {
 ///
 /// [`bytes()`]: trait.Read.html#method.bytes
 #[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
 pub struct Bytes<R> {
     inner: R,
 }
@@ -1623,6 +1635,7 @@ impl<R: Read> Iterator for Bytes<R> {
 /// [chars]: trait.Read.html#method.chars
 #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
            issue = "27802")]
+#[derive(Debug)]
 pub struct Chars<R> {
     inner: R,
 }
@@ -1712,6 +1725,7 @@ impl fmt::Display for CharsError {
 ///
 /// [split]: trait.BufRead.html#method.split
 #[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
 pub struct Split<B> {
     buf: B,
     delim: u8,
@@ -1743,6 +1757,7 @@ impl<B: BufRead> Iterator for Split<B> {
 ///
 /// [lines]: trait.BufRead.html#method.lines
 #[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
 pub struct Lines<B> {
     buf: B,
 }