Deny warnings in doc tests

This magical attribute is grossly underdocumented, but it works. And
it's literally the only thing that makes this work the way I want it to.

src/lib.rs

@@ -1,3 +1,6 @@
+// Deny warnings inside doc tests / examples
+#![doc(test(attr(deny(warnings))))]
+
 #[cfg_attr(test, macro_use)]
 extern crate hlist_macro;
 

src/lua.rs

@@ -436,6 +436,7 @@ impl<'lua> Table<'lua> {
     ///
     /// for pair in globals.pairs::<Value, Value>() {
     ///     let (key, value) = pair?;
+    /// #   let _ = (key, value);   // used
     ///     // ...
     /// }
     /// # Ok(())