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/libcollectionstest/str.rs

@@ -813,6 +813,37 @@ fn test_bytes_revator() {
     }
 }
 
+#[test]
+fn test_bytesator_nth() {
+    let s = "ศไทย中华Việt Nam";
+    let v = [
+        224, 184, 168, 224, 185, 132, 224, 184, 151, 224, 184, 162, 228,
+        184, 173, 229, 141, 142, 86, 105, 225, 187, 135, 116, 32, 78, 97,
+        109
+    ];
+
+    let mut b = s.bytes();
+    assert_eq!(b.nth(2).unwrap(), v[2]);
+    assert_eq!(b.nth(10).unwrap(), v[10]);
+    assert_eq!(b.nth(200), None);
+}
+
+#[test]
+fn test_bytesator_count() {
+    let s = "ศไทย中华Việt Nam";
+
+    let b = s.bytes();
+    assert_eq!(b.count(), 28)
+}
+
+#[test]
+fn test_bytesator_last() {
+    let s = "ศไทย中华Việt Nam";
+
+    let b = s.bytes();
+    assert_eq!(b.last().unwrap(), 109)
+}
+
 #[test]
 fn test_char_indicesator() {
     let s = "ศไทย中华Việt Nam";

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")]