Add new acceptance test scenario (018)

```
  Error: examples/acceptance_tests/018/lib/test.ak

    × or_else_2 failed
    help: ┍━ left ━━━━━━━━━━━┑
          │ (con data #182a) │
          ┕━━━━━━━━━━━━━━━━━━┙

          should be equal to

          ┍━ right ━━━━━━━━━━┑
          │ (con integer 42) │
          ┕━━━━━━━━━━━━━━━━━━┙
  ```

examples/acceptance_tests/018/aiken.toml

@@ -0,0 +1,2 @@
+name = "acceptance_test_018"
+version = "0.0.0"

examples/acceptance_tests/018/lib/test.ak

@@ -0,0 +1,10 @@
+pub fn or_else(opt: Option<a>, default: a) -> a {
+  when opt is {
+    None -> default
+    Some(a) -> a
+  }
+}
+
+test or_else_2() {
+  or_else(Some(42), 14) == 42
+}