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/binder.rs

@@ -197,11 +197,6 @@ impl Binder {
                 }
             }
 
-            UntypedKind::Parameter(_) => {
-                diag.push_error("Unexpected 'Parameter' node in general binder context. This should be handled via 'bind_pattern'.", Some(node.identity.clone()));
-                self.make_node(node.identity.clone(), BoundKind::Error)
-            }
-
             UntypedKind::FieldAccessor(k) => {
                 self.make_node(node.identity.clone(), BoundKind::FieldAccessor(*k))
             }
@@ -230,7 +225,7 @@ impl Binder {
 
             UntypedKind::Def { target, value } => {
                 // Special case: Single identifier (to support recursion)
-                if let UntypedKind::Parameter(ref name) = target.kind {
+                if let UntypedKind::Identifier(ref name) = target.kind {
                     let addr_opt = self.declare_variable(
                         name,
                         node.identity.clone(), // Identity of the Def node
@@ -543,7 +538,7 @@ impl Binder {
         diag: &mut Diagnostics,
     ) -> BoundNode {
         match &node.kind {
-            UntypedKind::Parameter(sym) => {
+            UntypedKind::Identifier(sym) => {
                 if let Some(addr) = self.declare_variable(sym, node.identity.clone(), kind, diag) {
                     self.make_node(
                         node.identity.clone(),