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/sys/unix/fast_thread_local.rs

@@ -12,6 +12,7 @@
 #![unstable(feature = "thread_local_internals", issue = "0")]
 
 use cell::{Cell, UnsafeCell};
+use fmt;
 use intrinsics;
 use ptr;
 
@@ -24,6 +25,12 @@ pub struct Key<T> {
     dtor_running: Cell<bool>,
 }
 
+impl<T> fmt::Debug for Key<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Key { .. }")
+    }
+}
+
 unsafe impl<T> ::marker::Sync for Key<T> { }
 
 impl<T> Key<T> {