Refactor Call to include is_tail flag

The `TailCall` variant has been merged into `Call` by adding an
`is_tail` boolean field. This simplifies the AST by reducing the number
of node variants and makes it easier to handle tail call optimization.
The `tco.rs` module is responsible for setting this flag when a call
occurs in tail position.

src/ast/compiler/tco.rs

@@ -14,26 +14,17 @@ impl TCO {
 
     fn transform<T: Clone>(node: Node<BoundKind<T>, T>, is_tail_position: bool) -> Node<BoundKind<T>, T> {
         match node.kind {
-            BoundKind::Call { callee, args } => {
+            BoundKind::Call { callee, args, .. } => {
                 let new_callee = Box::new(Self::transform(*callee, false));
                 let new_args = Box::new(Self::transform(*args, false));
                 
-                if is_tail_position {
-                    Node {
-                        kind: BoundKind::TailCall {
-                            callee: new_callee,
-                            args: new_args,
-                        },
-                        ..node
-                    }
-                } else {
-                    Node {
-                        kind: BoundKind::Call {
-                            callee: new_callee,
-                            args: new_args,
-                        },
-                        ..node
-                    }
+                Node {
+                    kind: BoundKind::Call {
+                        callee: new_callee,
+                        args: new_args,
+                        is_tail: is_tail_position,
+                    },
+                    ..node
                 }
             },