robot-name: use is_ascii_* functions

https://rust-lang.github.io/rust-clippy/master/index.html#manual_is_ascii_check

Using the built-in functions is more readable and makes it clear that
it’s not a specific subset of characters, but all ASCII
(lowercase|uppercase|digit) characters.

Author: petertseng

exercises/practice/robot-name/tests/robot-name.rs

@@ -3,11 +3,11 @@ use robot_name as robot;
 fn assert_name_matches_pattern(n: &str) {
     assert!(n.len() == 5, "name is exactly 5 characters long");
     assert!(
-        n[0..2].chars().all(|c| ('A'..='Z').contains(&c)),
+        n[0..2].chars().all(|c| c.is_ascii_uppercase()),
         "name starts with 2 uppercase letters"
     );
     assert!(
-        n[2..].chars().all(|c| ('0'..='9').contains(&c)),
+        n[2..].chars().all(|c| c.is_ascii_digit()),
         "name ends with 3 numbers"
     );
 }