Refactor: Use Rc/RefCell for shared mutable state

This commit replaces `Arc<Mutex<T>>` with `Rc<RefCell<T>>` for managing
shared mutable state within the AST and VM. This change is primarily an
internal refactoring to leverage Rust's standard library more
effectively for single-threaded scenarios, improving performance by
avoiding the overhead of mutexes.

The following types and their usage have been updated:
- `Environment.global_names` and `Environment.global_values`
- `Binder.globals`
- `bound_nodes::BoundKind::Lambda.body` (now `Rc<Node>`)
- `ast::types::NodeIdentity` (now `Rc<NodeIdentity>`)
- `ast::types::Value::List`, `Value::Record`, `Value::Function`,
  `Value::Object`, `Value::Cell`
- `ast::nodes::Scope` and `ast::nodes::Context`
- `ast::vm::Closure.function_node` and `Closure.upvalues`
- `ast::vm::VM.globals`

This change does not alter the external behavior of the library but
streamlines internal data management.

src/integration_test.rs

@@ -1,6 +1,7 @@
 #[cfg(test)]
 mod tests {
-    use std::sync::{Arc, Mutex};
+    use std::rc::Rc;
+    use std::cell::RefCell;
     use crate::ast::parser::Parser;
     use crate::ast::compiler::binder::Binder;
     use crate::ast::vm::VM;
@@ -74,11 +75,11 @@ mod tests {
         let mut parser = Parser::new(source).expect("Failed to create parser");
         let ast = parser.parse_expression().expect("Failed to parse");
 
-        let globals = Arc::new(Mutex::new(std::collections::HashMap::new()));
+        let globals = Rc::new(RefCell::new(std::collections::HashMap::new()));
         let mut binder = Binder::new(globals.clone());
         let bound_ast = binder.bind(&ast).expect("Failed to bind");
 
-        let vm_globals = Arc::new(Mutex::new(Vec::new()));
+        let vm_globals = Rc::new(RefCell::new(Vec::new()));
         let mut vm = VM::new(vm_globals);
         let result = vm.run(&bound_ast);