Refactor scope handling and immutability

Introduces `fixed_scope_idx` to `Binder` and `Environment` to track
immutable scopes.
This allows enforcing that definitions (`def`) cannot occur in scopes
that are considered fixed or immutable, such as the global scope or the
initial bootstrap scope.
The `FunctionCompiler` is updated to use `fixed_scope_idx` and `is_root`
to determine scope immutability.

src/ast/compiler/macros.rs

@@ -778,7 +778,7 @@ mod tests {
         let globals = Rc::new(RefCell::new(global_names));
 
         let mut diag = crate::ast::diagnostics::Diagnostics::new();
-        let result = Binder::bind_root(globals, &expanded, &mut diag);
+        let result = Binder::bind_root(globals, 0, &expanded, &mut diag);
         assert!(
             result.is_ok(),
             "Should find global '*' Error: {:?}",
@@ -803,7 +803,7 @@ mod tests {
 
         let globals = Rc::new(RefCell::new(HashMap::new()));
         let mut diag = crate::ast::diagnostics::Diagnostics::new();
-        let result = Binder::bind_root(globals, &expanded, &mut diag);
+        let result = Binder::bind_root(globals, 0, &expanded, &mut diag);
         assert!(result.is_ok(), "Hygiene failed: {:?}", result.err());
     }
 
@@ -824,7 +824,7 @@ mod tests {
 
         let globals = Rc::new(RefCell::new(HashMap::new()));
         let mut diag = crate::ast::diagnostics::Diagnostics::new();
-        let result = Binder::bind_root(globals, &expanded, &mut diag);
+        let result = Binder::bind_root(globals, 0, &expanded, &mut diag);
         assert!(
             result.is_ok(),
             "Explicit Hygiene with Backticks failed: {:?}",