Refactor binder to simplify root scope logic

The previous implementation of the binder had a slightly complex way of
handling the root scope. This commit simplifies that logic by directly
checking if the current scope is the root scope and the scope stack has
only one element. This makes the code more readable and maintainable.

src/ast/compiler/binder.rs

@@ -70,8 +70,7 @@ impl FunctionCompiler {
         identity: Identity,
         globals: &Rc<RefCell<HashMap<Symbol, (GlobalIdx, Identity)>>>,
     ) -> Result<Address, String> {
-        match self.kind {
+        if self.kind == ScopeKind::Root && self.scopes.len() == 1 {
-            ScopeKind::Root => {
             let current_scope = self.scopes.last_mut().unwrap();
             if current_scope.locals.contains_key(name) {
                 return Err(format!(
@@ -101,8 +100,7 @@ impl FunctionCompiler {
                 },
             );
             Ok(addr)
-            }
+        } else {
-            ScopeKind::Local => {
             let current_scope = self.scopes.last_mut().unwrap();
             if current_scope.locals.contains_key(name) {
                 return Err(format!(
@@ -123,7 +121,6 @@ impl FunctionCompiler {
             Ok(Address::Local(slot))
         }
     }
-    }
 
     fn resolve_local(&self, sym: &Symbol) -> Option<LocalInfo> {
         for scope in self.scopes.iter().rev() {