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`.

examples/pipeline_script_ticker.myc

@@ -1,5 +1,5 @@
-;; Benchmark: 3.2us
-;; Benchmark-Repeat: 613
+;; Benchmark: 2.3us
+;; Benchmark-Repeat: 870
 ;; Output: <StreamNode>
 
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