Introduce closure AST node

The `Closure` struct has been moved from `ast/vm.rs` to a new module
`ast/closure.rs`. This improves the organization of the AST nodes. The
`Closure` struct represents a compiled function body along with its
captured upvalues, which is a key feature for Myc Script.

src/ast/compiler/optimizer/engine.rs

@@ -3,7 +3,7 @@ use crate::ast::nodes::{
     IdentifierBinding, LambdaBinding, Node, NodeKind, UpvalueIdx, VirtualId,
 };
 use crate::ast::types::{Purity, Value};
-use crate::ast::vm::Closure;
+use crate::ast::closure::Closure;
 use std::cell::RefCell;
 use std::rc::Rc;
 

src/ast/compiler/optimizer/inliner.rs

@@ -2,7 +2,7 @@ use crate::ast::nodes::{
     Address, AnalyzedNode, IdentifierBinding, NodeKind, VirtualId,
 };
 use crate::ast::types::{Purity, Value};
-use crate::ast::vm::Closure;
+use crate::ast::closure::Closure;
 use std::cell::RefCell;
 use std::collections::HashSet;
 use std::rc::Rc;

src/ast/compiler/optimizer/utils.rs

@@ -3,7 +3,7 @@ use crate::ast::nodes::{
     NodeKind, VirtualId,
 };
 use crate::ast::types::{Identity, Value};
-use crate::ast::vm::Closure;
+use crate::ast::closure::Closure;
 use std::collections::HashSet;
 
 // --- PathTracker ---