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/ast/lexer.rs

@@ -1,6 +1,6 @@
 use std::iter::Peekable;
 use std::str::Chars;
-use std::sync::Arc;
+use std::rc::Rc;
 use crate::ast::types::SourceLocation;
 
 #[derive(Debug, Clone, PartialEq)]
@@ -9,11 +9,11 @@ pub enum TokenKind {
     LeftBracket, RightBracket,
     LeftBrace, RightBrace,
     Quote, Backtick, Tilde, At,
-    Identifier(Arc<str>),
-    Keyword(Arc<str>),
+    Identifier(Rc<str>),
+    Keyword(Rc<str>),
     Integer(i64),
     Float(f64),
-    String(Arc<str>),
+    String(Rc<str>),
     EOF,
 }
 
@@ -65,9 +65,9 @@ impl<'a> Lexer<'a> {
             '@' => TokenKind::At,
             ':' => {
                 let id = self.read_while(|c| !c.is_whitespace() && !"()[]{}\"'`~@;".contains(c));
-                TokenKind::Keyword(Arc::from(id))
+                TokenKind::Keyword(Rc::from(id))
             }
-            '"' => TokenKind::String(Arc::from(self.read_string()?)),
+            '"' => TokenKind::String(Rc::from(self.read_string()?)),
             c if c.is_ascii_digit() || (c == '-' && self.input.peek().is_some_and(|p| p.is_ascii_digit())) => {
                 let mut num_str = String::from(c);
                 while let Some(&next) = self.peek() {
@@ -88,7 +88,7 @@ impl<'a> Lexer<'a> {
             c if is_ident_start(c) => {
                 let mut id = String::from(c);
                 id.push_str(&self.read_while(is_ident_char));
-                TokenKind::Identifier(Arc::from(id))
+                TokenKind::Identifier(Rc::from(id))
             }
             _ => return Err(format!("Unexpected character: {} at {}:{}", char, self.line, self.col - 1)),
         };