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/process.rs

@@ -114,6 +114,17 @@ impl IntoInner<imp::Process> for Child {
     fn into_inner(self) -> imp::Process { self.handle }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Child {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("Child")
+            .field("stdin", &self.stdin)
+            .field("stdout", &self.stdout)
+            .field("stderr", &self.stderr)
+            .finish()
+    }
+}
+
 /// A handle to a child process's stdin. This struct is used in the [`stdin`]
 /// field on [`Child`].
 ///
@@ -149,6 +160,13 @@ impl FromInner<AnonPipe> for ChildStdin {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for ChildStdin {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("ChildStdin { .. }")
+    }
+}
+
 /// A handle to a child process's stdout. This struct is used in the [`stdout`]
 /// field on [`Child`].
 ///
@@ -183,6 +201,13 @@ impl FromInner<AnonPipe> for ChildStdout {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for ChildStdout {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("ChildStdout { .. }")
+    }
+}
+
 /// A handle to a child process's stderr. This struct is used in the [`stderr`]
 /// field on [`Child`].
 ///
@@ -217,6 +242,13 @@ impl FromInner<AnonPipe> for ChildStderr {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for ChildStderr {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("ChildStderr { .. }")
+    }
+}
+
 /// A process builder, providing fine-grained control
 /// over how a new process should be spawned.
 ///
@@ -622,6 +654,13 @@ impl FromInner<imp::Stdio> for Stdio {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Stdio {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Stdio { .. }")
+    }
+}
+
 /// Describes the result of a process after it has terminated.
 #[derive(PartialEq, Eq, Clone, Copy, Debug)]
 #[stable(feature = "process", since = "1.0.0")]