Add implementations of last in terms of next_back on a bunch of DoubleEndedIterators.

r?Manishearth

src/libstd/path.rs

@@ -886,6 +886,11 @@ impl<'a> Iterator for Iter<'a> {
     fn next(&mut self) -> Option<&'a OsStr> {
         self.inner.next().map(Component::as_os_str)
     }
+
+    #[inline]
+    fn last(mut self) -> Option<&'a OsStr> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -949,6 +954,11 @@ impl<'a> Iterator for Components<'a> {
         }
         None
     }
+
+    #[inline]
+    fn last(mut self) -> Option<Self::Item> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]