2021 day8/rust: prettify calculation of output number

Now that `digits` is a fixed-size array, both methods get unrolled the
same and there is no performance difference.

2021/day8/day8_rs/src/v2.rs

@@ -42,14 +42,6 @@ pub fn unscramble(line: &str) -> LineResult {
 
     LineResult {
         unique_digits: digits.iter().filter(|d| [1, 4, 7, 8].contains(d)).count(),
-        number: digits
-            .iter()
-            .rev()
-            .scan(1, |mul, &d| {
-                let ret = Some(d * *mul);
-                *mul *= 10;
-                ret
-            })
-            .sum(),
+        number: digits.iter().fold(0, |acc, d| acc * 10 + d),
     }
 }