Refactor series implementation into its own module

This commit reorganizes the series-related code by moving the
`RingBuffer`, `ScalarSeries`, `ValueSeries`, `RecordSeries`,
`SeriesView`, and `SharedSeries` structs into a new
`src/ast/rtl/series/data.rs` file.

The `src/ast/rtl/series/mod.rs` file now contains the module
declaration, re-exports, and the `register` function for registering
series-related native functions with the environment. This improves code
organization and maintainability.

src/ast/rtl/prelude.myc

@@ -0,0 +1,30 @@
+;; Myc System Library
+;; Standard macros and core utilities.
+
+(do
+  (macro while [cond body]
+    `((fn [] (if ~cond
+               (do ~body (again))
+             )))
+  )
+
+  (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
+       (def data (series ~lookback ~type))
+       (pipe [~src] (fn [s] (do (push data s))))
+       data
+     )
+  )
+)