Simpler character counting (#1419)

`str.chars().map(|_| 1).sum()` is just `str.chars().count()`.

src/unsafe/calling-unsafe-functions.md

@@ -23,6 +23,6 @@ fn main() {
 }
 
 fn count_chars(s: &str) -> usize {
-    s.chars().map(|_| 1).sum()
+    s.chars().count()
 }
 ```