Add as_nanos function to duration

Author: fintelia

src/libcore/time.rs

@@ -263,6 +263,22 @@ impl Duration {
     #[inline]
     pub fn subsec_nanos(&self) -> u32 { self.nanos }
 
+    /// Returns the total number of nanoseconds contained by this `Duration`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::time::Duration;
+    ///
+    /// let duration = Duration::new(5, 730023852);
+    /// assert_eq!(duration.as_nanos(), 5730023852);
+    /// ```
+    #[unstable(feature = "duration_nanos", issue = "0")]
+    #[inline]
+    pub fn as_nanos(&self) -> u128 {
+        self.secs as u128 * 1000_000_000 + self.nanos as u128
+    }
+
     /// Checked `Duration` addition. Computes `self + other`, returning [`None`]
     /// if overflow occurred.
     ///