Auto merge of #37888 - bluss:chars-count, r=alexcrichton

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.

benchmark descriptions and results for two configurations:

- ascii: ascii text
- cy: cyrillic text
- jp: japanese text
- words ascii: counting each split_whitespace item from the ascii text
- words jp: counting each split_whitespace item from the jp text

```
x86-64 rustc -Copt-level=3
 name               orig_ ns/iter      cmov_ ns/iter      diff ns/iter   diff %
 count_ascii        1,453 (1755 MB/s)  1,398 (1824 MB/s)           -55   -3.79%
 count_cy           5,990 (856 MB/s)   2,545 (2016 MB/s)        -3,445  -57.51%
 count_jp           3,075 (1169 MB/s)  1,772 (2029 MB/s)        -1,303  -42.37%
 count_words_ascii  4,157 (521 MB/s)   1,797 (1205 MB/s)        -2,360  -56.77%
 count_words_jp     3,337 (1071 MB/s)  1,772 (2018 MB/s)        -1,565  -46.90%

x86-64 rustc -Ctarget-feature=+avx -Copt-level=3
 name               orig_ ns/iter      cmov_ ns/iter      diff ns/iter   diff %
 count_ascii        1,444 (1766 MB/s)  763 (3343 MB/s)            -681  -47.16%
 count_cy           5,871 (874 MB/s)   1,527 (3360 MB/s)        -4,344  -73.99%
 count_jp           2,874 (1251 MB/s)  1,073 (3351 MB/s)        -1,801  -62.67%
 count_words_ascii  4,131 (524 MB/s)   1,871 (1157 MB/s)        -2,260  -54.71%
 count_words_jp     3,253 (1099 MB/s)  1,331 (2686 MB/s)        -1,922  -59.08%
```

I briefly explored a more involved blocked algorithm (looking at 8 or more bytes at a time),
but the code in this PR was always winning `count_words_ascii` in particular (counting
many small strings); this solution is an improvement without tradeoffs.

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();
@@ -507,6 +518,11 @@ 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()