Refactor UntypedKind::Parameter to Identifier

The `Parameter` kind in `UntypedKind` was only used for identifiers that
were being declared or referenced. This commit renames it to
`Identifier` and updates all the necessary code to reflect this change.
This simplifies the AST and makes it more consistent.

Additionally, a new macro `repeat` has been added to
`src/ast/system.myc`.

src/ast/environment.rs

@@ -381,7 +381,7 @@ impl Environment {
     fn discover_globals(&self, node: &Node<UntypedKind>) {
         match &node.kind {
             UntypedKind::Def { target, .. } => {
-                if let UntypedKind::Parameter(sym) = &target.kind {
+                if let UntypedKind::Identifier(sym) = &target.kind {
                     let mut names = self.global_names.borrow_mut();
                     if !names.contains_key(sym) {
                         let idx = GlobalIdx(names.len() as u32);
@@ -391,12 +391,11 @@ impl Environment {
             }
             UntypedKind::MacroDecl { name, params, body } => {
                 let mut registry = self.macro_registry.borrow_mut();
-                
+
                 fn extract_names(node: &Node<UntypedKind>) -> Vec<Rc<str>> {
                     match &node.kind {
-                        UntypedKind::Parameter(sym) => vec![sym.name.clone()],
-                        UntypedKind::Tuple { elements } => {
-                            elements.iter().flat_map(extract_names).collect()
+                        UntypedKind::Identifier(sym) => vec![sym.name.clone()],
+                        UntypedKind::Tuple { elements } => {                            elements.iter().flat_map(extract_names).collect()
                         }
                         _ => vec![],
                     }