Fix formatter for unary operation

Was wrongly converting any unary operation into '!'

crates/aiken-lang/src/tests/format.rs

@@ -14,7 +14,7 @@ fn assert_fmt(src: &str, expected: &str) {
     let (module2, extra2) = parser::module(&out, ModuleKind::Lib).unwrap();
     let mut out2 = String::new();
     format::pretty(&mut out2, module2, extra2, &out);
-    assert_eq!(out, out2);
+    assert!(out == out2, "formatting isn't idempotent");
 }
 
 #[test]
@@ -197,3 +197,20 @@ fn test_format_imports() {
 
     assert_fmt(src, expected)
 }
+
+#[test]
+fn test_negate() {
+    let src = indoc! {r#"
+        fn foo() {
+            - 42
+        }
+    "#};
+
+    let expected = indoc! {r#"
+        fn foo() {
+          -42
+        }
+    "#};
+
+    assert_fmt(src, expected)
+}