Optimise Chars::last()

The default implementation of last() goes through the entire iterator
but that's not needed here.

src/libcollectionstest/str.rs

@@ -814,6 +814,14 @@ fn test_iterator_clone() {
     assert!(it.clone().zip(it).all(|(x,y)| x == y));
 }
 
+#[test]
+fn test_iterator_last() {
+    let s = "ศไทย中华Việt Nam";
+    let mut it = s.chars();
+    it.next();
+    assert_eq!(it.last(), Some('m'));
+}
+
 #[test]
 fn test_bytesator() {
     let s = "ศไทย中华Việt Nam";

src/libcore/str/mod.rs

@@ -432,6 +432,12 @@ impl<'a> Iterator for Chars<'a> {
         // `isize::MAX` (that's well below `usize::MAX`).
         ((len + 3) / 4, Some(len))
     }
+
+    #[inline]
+    fn last(mut self) -> Option<char> {
+        // No need to go through the entire string.
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]