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/system.myc

@@ -8,6 +8,17 @@
              )))
   )
 
+  (macro repeat [var limit body]
+    `((fn [~var __limit]
+        (if (< ~var __limit)
+          (do
+            ~body
+            (again (+ ~var 1) __limit)
+          )
+        ))
+      0 ~limit)
+  )
+
   ;; Creates a stateful cache (Series) from a stateless stream.
   (macro cache [lookback type src]
     `(do