Refactor: Use Symbol for identifiers and macro hygiene

Introduces a `Symbol` struct to represent identifiers, incorporating
macro hygiene context. Updates various parts of the AST compiler and
environment to use `Symbol` instead of raw `Rc<str>` for identifiers,
improving robustness for macro expansions.

Also includes:
- Adds a new example `macro_hygiene.myc`.
- Updates `.gitignore`.
- Refactors `MacroExpander` to use `ExpansionState` for cleaner template
  expansion.
- Adjusts `VM::eval_observed` to ensure `after_eval` is called
  correctly.
- Resets `Environment` for each test run and compilation/dumping in
  `main.rs`.

src/ast/nodes.rs

@@ -3,6 +3,27 @@ use std::fmt::Debug;
 use std::any::Any;
 use crate::ast::types::{Identity, Value, Object};
 
+/// A name with an optional context for macro hygiene.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Symbol {
+    pub name: Rc<str>,
+    /// Points to the identity of the Expansion node if this symbol 
+    /// was created/referenced inside a macro expansion.
+    pub context: Option<Identity>,
+}
+
+impl From<Rc<str>> for Symbol {
+    fn from(name: Rc<str>) -> Self {
+        Self { name, context: None }
+    }
+}
+
+impl From<&str> for Symbol {
+    fn from(name: &str) -> Self {
+        Self { name: Rc::from(name), context: None }
+    }
+}
+
 /// A generic AST Node wrapper to preserve identity and metadata
 #[derive(Debug, Clone)]
 pub struct Node<K, T = ()> {
@@ -36,14 +57,14 @@ impl Clone for Box<dyn CustomNode> {
 pub enum UntypedKind {
     Nop,
     Constant(Value),
-    Identifier(Rc<str>),
+    Identifier(Symbol),
     If {
         cond: Box<Node<UntypedKind>>,
         then_br: Box<Node<UntypedKind>>,
         else_br: Option<Box<Node<UntypedKind>>>,
     },
     Def {
-        name: Rc<str>,
+        name: Symbol,
         value: Box<Node<UntypedKind>>,
     },
     Assign {
@@ -51,7 +72,7 @@ pub enum UntypedKind {
         value: Box<Node<UntypedKind>>,
     },
     Lambda {
-        params: Vec<Rc<str>>,
+        params: Vec<Symbol>,
         body: Rc<Node<UntypedKind>>,
     },
     Call {
@@ -69,8 +90,8 @@ pub enum UntypedKind {
     },
     /// A macro declaration that can be expanded at compile time.
     MacroDecl {
-        name: Rc<str>,
-        params: Vec<Rc<str>>,
+        name: Symbol,
+        params: Vec<Symbol>,
         body: Box<Node<UntypedKind>>,
     },
     /// A template for AST nodes, allowing for substitutions. (Quasiquote)