auto merge of #19369 : seanmonstar/rust/cow-str, r=alexcrichton

This implementation existed on MaybeOwned, but has been lost in the
transition to Cows. Let's put it back.

@aturon r?

src/libcollections/str.rs

@@ -656,6 +656,13 @@ Section: CowString
 /// A clone-on-write string
 pub type CowString<'a> = Cow<'a, String, str>;
 
+impl<'a> Str for CowString<'a> {
+    #[inline]
+    fn as_slice<'b>(&'b self) -> &'b str {
+        (**self).as_slice()
+    }
+}
+
 /*
 Section: Trait implementations
 */