Fix formatter discarding nul bytes.

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

@@ -20,6 +20,18 @@ fn format_simple_module() {
     );
 }
 
+#[test]
+fn format_nul_byte() {
+    assert_format!(
+        r#"
+        fn label(str: String) -> Void {
+          str
+            |> builtin.append_string(@"\0", _)
+            |> builtin.debug(Void)
+        }"#
+    );
+}
+
 #[test]
 fn format_g1_element_constant() {
     assert_format!(

crates/aiken-lang/src/tests/snapshots/format_nul_byte.snap

@@ -0,0 +1,9 @@
+---
+source: crates/aiken-lang/src/tests/format.rs
+description: "Code:\n\nfn label(str: String) -> Void {\n  str\n    |> builtin.append_string(@\"\\0\", _)\n    |> builtin.debug(Void)\n}"
+---
+fn label(str: String) -> Void {
+  str
+    |> builtin.append_string(@"\0", _)
+    |> builtin.debug(Void)
+}