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.

examples/sma.myc

@@ -1,22 +1,20 @@
 #use rtl
 
 (do
-  (def src (create-random-ohlc 42 20000))
+  (def src (create-random-ohlc 42 200))
 
   (macro printx [s] `(fn [x] (print ~s x)))
 
-;  (pipe src print)
-;  (pipe [(pipe [(.close src)] (SMA 20))] (printx "sma20="))
-;  (pipe [(pipe [(.close src)] (SMA 5))] (printx "sma5="))
-;  (pipe [(pipe [(.close src)] (SMA 100))] (printx "sma100="))
-
-(macro cache [lookback type src] `(do (def data (series ~lookback ~type)) (pipe [~src] (fn [s] (do (push data s)))) data))
-
-
-
-;  (def bars (series 20 :float))
-;  (pipe [src] (fn [s] (do (push bars (.close s)))))
-
-(cache 20 :float src)
-  
-)
+  (pipe src print)
+  (pipe [(pipe [(.close src)] (SMA 20))] (printx "sma20="))
+  (pipe [(pipe [(.close src)] (SMA 5))] (printx "sma5="))
+  (pipe [(pipe [(.close src)] (SMA 100))] (printx "sma100="))
+
+  (macro cache [lookback type src] `(do (def data (series ~lookback ~type)) (pipe [~src] (fn [s] (do (push data s)))) data))
+
+;  (def bars (series 20 :float))
+;  (pipe [src] (fn [s] (do (push bars (.close s)))))
+
+  (cache 20 :float src)
+
+)