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/compiler/upvalues.rs

@@ -1,6 +1,5 @@
 use std::collections::{HashMap, HashSet};
-use std::rc::Rc;
-use crate::ast::nodes::{Node, UntypedKind};
+use crate::ast::nodes::{Node, UntypedKind, Symbol};
 use crate::ast::types::Identity;
 
 /// Analyzes the AST to find which lambdas capture which variable declarations.
@@ -22,15 +21,15 @@ impl UpvalueAnalyzer {
 
     fn visit(
         node: &Node<UntypedKind>, 
-        scopes: &mut Vec<HashMap<Rc<str>, Identity>>, 
+        scopes: &mut Vec<HashMap<Symbol, Identity>>, 
         capture_map: &mut HashMap<Identity, HashSet<Identity>>,
         current_lambda: Option<Identity>
     ) {
         match &node.kind {
-            UntypedKind::Identifier(name) => {
+            UntypedKind::Identifier(sym) => {
                 // Resolve name in scope stack (from inner to outer)
                 for (depth, scope) in scopes.iter().rev().enumerate() {
-                    if let Some(decl_id) = scope.get(name) {
+                    if let Some(decl_id) = scope.get(sym) {
                         if depth > 0 {
                             // Captured from an outer scope!
                             if let Some(lambda_id) = &current_lambda {