Remove unused system library and simplify environment initialization

The `system.myc` file, containing macros and core utilities, has been
removed as its functionality is now handled by an embedded string. This
simplifies the build process and eliminates a file that was no longer
strictly necessary.

Additionally, the `with_cwd()` method on the `Environment` has been
removed. The environment now automatically adds the current working
directory and its `rtl` subdirectory to the search paths during
initialization. This streamlines the setup for file-based usage and
ensures standard search paths are always considered.

rtl/system.myc

@@ -1,19 +0,0 @@
-;; 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
-     )
-  )
-)