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/main.rs

@@ -87,6 +87,9 @@ impl CompilerApp {
         let start_total = std::time::Instant::now();
         self.trace_logs.clear();
         
+        // Reset environment for a fresh run
+        self.env = Environment::new();
+        
         let res = (|| -> Result<(myc::ast::types::Value, std::time::Duration, std::time::Duration, std::time::Duration), String> {
             let start_compile = std::time::Instant::now();
             let compiled = self.env.compile(&self.source_code)?;
@@ -110,11 +113,10 @@ impl CompilerApp {
                 }
                 
                 for line in &mut logs {
-                    if line.len() > 255 {
-                        if let Some((idx, _)) = line.char_indices().nth(255) {
+                    if line.len() > 255
+                        && let Some((idx, _)) = line.char_indices().nth(255) {
                             line.truncate(idx);
                             line.push_str("...");
-                        }
                     }
                 }
 
@@ -154,6 +156,7 @@ impl CompilerApp {
     }
 
     fn dump_ast(&mut self) {
+        self.env = Environment::new();
         match self.env.dump_ast(&self.source_code) {
             Ok(dump) => {
                 self.output_log = format!(
@@ -367,6 +370,7 @@ impl eframe::App for CompilerApp {
                 // Log Output Area
                 egui::ScrollArea::vertical()
                     .id_salt("output_log_scroll")
+                    .auto_shrink([false, true]) // Don't shrink horizontally, fill width
                     .show(ui, |ui| {
                         match self.active_tab {
                             AppTab::Output => {
@@ -379,14 +383,12 @@ impl eframe::App for CompilerApp {
                                 );
                             },
                             AppTab::Trace => {
-                                egui::ScrollArea::vertical()
-                                    .id_salt("trace_log_scroll")
-                                    .show(ui, |ui| {
-                                        ui.style_mut().override_text_style = Some(egui::TextStyle::Monospace);
-                                        for line in &self.trace_logs {
-                                            ui.label(line);
-                                        }
-                                    });
+                                ui.style_mut().override_text_style = Some(egui::TextStyle::Monospace);
+                                ui.with_layout(egui::Layout::top_down_justified(egui::Align::LEFT), |ui| {
+                                    for line in &self.trace_logs {
+                                        ui.label(line);
+                                    }
+                                });
                             }
                         }
                     });