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/parser.rs

@@ -358,7 +358,7 @@ impl<'a> Parser<'a> {
                 };
                 Node {
                     identity: NodeIdentity::new(token.location),
-                    kind: UntypedKind::Parameter(sym),
+                    kind: UntypedKind::Identifier(sym),
                     ty: (),
                 }
             }