Refactor: Use Value::Closure enum variant

Previously, compiled closures were stored as `Value::Object` and then
downcasted. This commit introduces a dedicated `Value::Closure` enum
variant to represent compiled closures. This improves type safety and
simplifies handling of closures throughout the compiler and VM.

This change involves:
- Updating type definitions to use `Value::Closure`.
- Adjusting downcasting logic to directly access the `Closure` struct.
- Modifying `run_with_args` and `run_with_args_observed` to expect
  `Value::Closure` for tail-call targets.
- Refining the handling of closures in the `Optimizer` and `Inliner` for
  better type clarity.

src/ast/environment.rs

@@ -668,13 +668,11 @@ impl Environment {
                 info.positional_count,
                 node.ty.stack_size,
             ));
-            let closure_obj: Rc<dyn Object> = closure;
-
             return Rc::new(NativeFunction {
                 purity: Purity::Impure,
                 func: Rc::new(move |args| {
                     let mut vm = VM::new(root_values.clone());
-                    match vm.run_with_args(closure_obj.clone(), args) {
+                    match vm.run_with_args(closure.clone(), args) {
                         Ok(v) => v,
                         Err(e) => panic!("Myc Runtime Error: {}", e),
                     }
@@ -692,12 +690,7 @@ impl Environment {
                     Err(e) => panic!("Myc Runtime Error: {}", e),
                 };
 
-                if let Value::Object(obj) = &res
-                    && obj
-                        .as_any()
-                        .downcast_ref::<Closure>()
-                        .is_some()
-                {
+                if let Value::Closure(obj) = &res {
                     match vm.run_with_args(obj.clone(), args) {
                         Ok(v) => v,
                         Err(e) => panic!("Myc Runtime Error (Closure): {}", e),
@@ -837,9 +830,7 @@ impl Environment {
         // 2. Execute the root closure immediately to get the actual script result.
         // All Myc scripts are wrapped in a parameterless lambda for consistency.
         let mut final_result = result;
-        if let Ok(Value::Object(obj)) = &final_result
-            && let Some(_closure) = obj.as_any().downcast_ref::<Closure>()
-        {
+        if let Ok(Value::Closure(obj)) = &final_result {
             final_result = vm.run_with_args_observed(&mut observer, obj.clone(), &[]);
         }