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/macro_issue.myc

@@ -0,0 +1,5 @@
+;; Output: 42
+(do
+  (macro t [x] `(fn [~x] ~x))
+  (def id (t a))
+  (id 42))

examples/pipeline_script_ticker.myc

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

examples/record_optimizations.myc

@@ -1,5 +1,5 @@
-;; Benchmark: 900.6us
-;; Benchmark-Repeat: 4
+;; Benchmark: 1.0ms
+;; Benchmark-Repeat: 3
 ;; Tests the effect of record inlining and field lookup optimization
 ;; Output: 10000
 (do

examples/record_specialization_test.myc

@@ -1,5 +1,5 @@
-;; Benchmark: 1.5us
-;; Benchmark-Repeat: 1348
+;; Benchmark: 1.1us
+;; Benchmark-Repeat: 1895
 ;; Test Record SoA specialization in Pipelines
 ;; Dank der neuen Spezialisierung wird hierfür im Hintergrund 
 ;; eine SharedRecordSeries mit SoA-Layout (Float-Puffer für mid und range) erstellt.