Add new (failing) acceptance test (007)

```
  Error: aiken::check

    × Checking
    ╰─▶ Not a function
     ╭─[./007/lib/test.ak:4:1]
   4 │     [(a, b), ..rest] -> {
   5 │       let (a_tail, b_tail) = unzip(rest)
     ·                                   ──────
   6 │       ([a, ..a_tail], [b, ..b_tail])
     ╰────
  ```

examples/acceptance_tests/007/aiken.toml

@@ -0,0 +1,2 @@
+name = "acceptance_test_007"
+version = "0.0.0"

examples/acceptance_tests/007/lib/test.ak

@@ -0,0 +1,9 @@
+pub fn unzip(xs: List<(a, b)>) -> (List<a>, List<b>) {
+  when xs is {
+    [] -> ([], [])
+    [(a, b), ..rest] -> {
+      let (a_tail, b_tail) = unzip(rest)
+      ([a, ..a_tail], [b, ..b_tail])
+    }
+  }
+}