problems(rs): add “258. Add Digits”

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/add-digits.rs

@@ -0,0 +1,13 @@
+impl Solution {
+    pub fn add_digits(num: i32) -> i32 {
+        if num == 0 {
+            return 0;
+        }
+
+        if num % 9 == 0 {
+            return 9;
+        }
+
+        num % 9
+    }
+}