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/binder.rs

@@ -233,7 +233,8 @@ impl Binder {
 
                 Ok(self.make_node(node.identity.clone(), BoundKind::Call { 
                     callee: Box::new(callee), 
-                    args: Box::new(args) 
+                    args: Box::new(args),
+                    is_tail: false 
                 }))
             },