use byte literals instead of casting char to u8 in exercise examples (#1017)

Author: coriolinus

exercises/minesweeper/example.rs

@@ -30,7 +30,7 @@ impl Board {
         if count == 0 {
             ' '
         } else {
-            (('0' as u8) + count) as char
+            (b'0' + count) as char
         }
     }
 }

exercises/simple-cipher/example.rs

@@ -5,7 +5,7 @@ pub fn encode_random(s: &str) -> (String, String) {
     let mut r = rand::thread_rng();
     let mut key = String::new();
     for _ in 0..100 {
-        key.push(char::from('a' as u8 + r.gen_range(0, 26)));
+        key.push(char::from(b'a' + r.gen_range(0, 26)));
     }
     let encoded = encode(&key, s);
     (key, encoded.unwrap())
@@ -34,8 +34,8 @@ fn shift(key: &str, s: &str, dir: i8) -> Option<String> {
     }
     for c in s.chars() {
         let shift = key_arr[i % key_arr.len()] as i8 - 'a' as i8;
-        let n = ((c as i8 - 'a' as i8 + dir * shift) % 26 + 26) % 26;
-        o.push(char::from('a' as u8 + n as u8));
+        let n = ((c as i8 - b'a' as i8 + dir * shift) % 26 + 26) % 26;
+        o.push(char::from(b'a' + n as u8));
         i += 1;
     }
     Some(o)