Refactor: Introduce system library

- Add `rtl/system.myc` as the system library, containing core utilities
  like `while` and `cache`.
- Remove the duplicate `prelude.myc` from `src/ast/rtl/`.
- Modify `Environment::collect_dependencies` to implicitly add `#use
  system` when necessary.
- Update `Environment::with_cwd` to also add the `rtl` subdirectory to
  search paths.
- Add `target/` to `.geminiignore` to exclude build artifacts.
- Adjust example `sma.myc` to use the new `cache` macro and reduce
  sample data size.
- Update `rtl/sma.myc` to correctly initialize the `history` series with
  its `length`.
- Add `preload_dependencies` calls to `dump_ast` and `run_script` for
  proper module loading.

rtl/sma.myc

@@ -2,7 +2,7 @@
  (def SMA
     (fn [length]
       (do
-        (def history (series :float))
+        (def history (series length :float))
         (def sum 0.0)
 
 		(fn [val]

rtl/system.myc

@@ -0,0 +1,19 @@
+;; Myc System Library
+;; Standard macros and core utilities.
+
+(do
+  (macro while [cond body]
+    `((fn [] (if ~cond
+               (do ~body (again))
+             )))
+  )
+
+  ;; 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
+     )
+  )
+)