Implement parser & formatter for Pair annotations.

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

@@ -792,3 +792,13 @@ fn superfluous_parens_in_binop() {
         "#
     );
 }
+
+#[test]
+fn format_pairs() {
+    assert_format!(
+        r#"
+        pub fn foo(x: Pair<Int, Int>) {
+            Pair(x.1st, x.2nd)
+        }"#
+    );
+}

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

@@ -0,0 +1,7 @@
+---
+source: crates/aiken-lang/src/tests/format.rs
+description: "Code:\n\npub fn foo(x: Pair<Int, Int>) {\n    Pair(x.1st, x.2nd)\n}"
+---
+pub fn foo(x: Pair<Int, Int>) {
+  Pair(x.1st, x.2nd)
+}