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/lambda_collector.rs

@@ -57,7 +57,7 @@ impl<'a> LambdaCollector<'a> {
                 self.visit(value);
             }
 
-            BoundKind::Call { callee, args } | BoundKind::TailCall { callee, args } => {
+            BoundKind::Call { callee, args, .. } => {
                 self.visit(callee);
                 self.visit(args);
             }