Add overrides to iterator methods for str::Bytes

Specifically, `count`, `last`, and `nth` are implemented to use the
methods of the underlying slice iterator.

Partially closes #24214.

src/libcore/str/mod.rs

@@ -387,6 +387,21 @@ impl<'a> Iterator for Bytes<'a> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.0.size_hint()
     }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.0.count()
+    }
+
+    #[inline]
+    fn last(self) -> Option<Self::Item> {
+        self.0.last()
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.0.nth(n)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]