add is_even and is_odd recursive functions (#1)

Reviewed-on: http://192.168.1.227:3000/sfrembling/timeshift/pulls/1
Co-authored-by: Shea Frembling <sfrembling@gmail.com>
Co-committed-by: Shea Frembling <sfrembling@gmail.com>

src/main.rs

@@ -1,3 +1,40 @@
 fn main() {
-    println!("Hello, world!");
+    let src = 12;
+    let result = is_even(src);
+    println!("{src} is even? {result}");
+}
+
+fn is_even(n: u32) -> bool {
+    match n {
+        0 => true,
+        1 => false,
+        _ => is_odd(n - 1),
+    }
+}
+
+fn is_odd(n: u32) -> bool {
+    match n {
+        0 => false,
+        1 => true,
+        _ => is_even(n - 1),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{is_even, is_odd};
+
+    #[test]
+    fn it_works() {
+        let src = 12;
+        let result = is_even(src);
+        assert!(result);
+    }
+
+    #[test]
+    fn it_works_again() {
+        let src = 5;
+        let result = is_odd(src);
+        assert!(result);
+    }
 }