str: Improve .chars().count()

Use a simpler loop to count the `char` of a string: count the
number of non-continuation bytes. Use `count += <conditional>` which the
compiler understands well and can apply loop optimizations to.

src/libcollectionstest/str.rs

@@ -767,6 +767,7 @@ fn test_iterator() {
         pos += 1;
     }
     assert_eq!(pos, v.len());
+    assert_eq!(s.chars().count(), v.len());
 }
 
 #[test]

src/libcore/str/mod.rs

@@ -424,6 +424,17 @@ impl<'a> Iterator for Chars<'a> {
         })
     }
 
+    #[inline]
+    fn count(self) -> usize {
+        // length in `char` is equal to the number of non-continuation bytes
+        let bytes_len = self.iter.len();
+        let mut cont_bytes = 0;
+        for &byte in self.iter {
+            cont_bytes += utf8_is_cont_byte(byte) as usize;
+        }
+        bytes_len - cont_bytes
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let len = self.iter.len();
@@ -501,6 +512,11 @@ impl<'a> Iterator for CharIndices<'a> {
         }
     }
 
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count()
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()