Rollup merge of rust-lang#30818 - sfackler:duration-hash, r=alexcrichton

tikue pointed out in IRC that this was missing.

Author: Manishearth

src/libstd/time/duration.rs

@@ -40,7 +40,7 @@ const MILLIS_PER_SEC: u64 = 1_000;
 /// let ten_millis = Duration::from_millis(10);
 /// ```
 #[stable(feature = "duration", since = "1.3.0")]
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
 pub struct Duration {
     secs: u64,
     nanos: u32, // Always 0 <= nanos < NANOS_PER_SEC