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/compiler/macros.rs

@@ -344,7 +344,7 @@ impl<E: MacroEvaluator> MacroExpander<E> {
 
     fn extract_param_names(&self, node: &Node<UntypedKind>) -> Result<Vec<Rc<str>>, String> {
         match &node.kind {
-            UntypedKind::Parameter(sym) => Ok(vec![sym.name.clone()]),
+            UntypedKind::Identifier(sym) => Ok(vec![sym.name.clone()]),
             UntypedKind::Tuple { elements } => {
                 let mut names = Vec::new();
                 for el in elements {
@@ -372,15 +372,6 @@ impl<E: MacroEvaluator> MacroExpander<E> {
                 })
             }
 
-            UntypedKind::Parameter(mut sym) => {
-                sym.context = Some(state.expansion_id.clone());
-                Ok(Node {
-                    identity: node.identity,
-                    kind: UntypedKind::Parameter(sym),
-                    ty: (),
-                })
-            }
-
             UntypedKind::Placeholder(inner) => {
                 // Break out of template for substitution/evaluation
                 if let UntypedKind::Identifier(ref sym) = inner.kind
@@ -654,11 +645,11 @@ mod tests {
             } = &exprs[1].kind
         {
             if let UntypedKind::Def { target, .. } = &result.kind {
-                if let UntypedKind::Parameter(sym) = &target.kind {
+                if let UntypedKind::Identifier(sym) = &target.kind {
                     assert_eq!(sym.context, Some(call.identity.clone()));
                     assert_eq!(sym.name.as_ref(), "y");
                 } else {
-                    panic!("Expected Parameter target, got {:?}", target.kind);
+                    panic!("Expected Identifier target, got {:?}", target.kind);
                 }
             } else {
                 panic!("Expected Def result, got {:?}", result.kind);