Add positional_count field to Lambda

This field is used for static optimization, determining if parameters
are purely positional.

src/ast/compiler/tco.rs

@@ -75,15 +75,16 @@ impl TCO {
                 }
             },
 
-            BoundKind::Lambda { params, upvalues, body } => {
+            BoundKind::Lambda { params, upvalues, body, positional_count } => {
                 // The body of a lambda is implicitly in tail position when the lambda is called.
                 let new_body = Rc::new(Self::transform((*body).clone(), true));
                 
                 Node {
                     kind: BoundKind::Lambda {
-                        params,
+                        params: params.clone(),
                         upvalues,
                         body: new_body,
+                        positional_count,
                     },
                     ..node
                 }