Refactor: Simplify series creation and infer element type

The `series` constructor now only accepts the `lookback` limit. The
element type is inferred by the type checker and implicitly added as a
schema argument during compilation. This simplifies the API and
centralizes type inference.

This change also includes:
- Updates to example scripts (`HMA.myc`, `sma.myc`, `soa_series.myc`) to
  reflect the new `series` signature.
- Modifications to the `TypeChecker` to handle the inferred schema
  injection.
- An addition of a regression test for type inference through nested
  closures with `series`.
- Removal of `#[allow(dead_code)]` from several `TypeChecker` methods as
  they are now used.
- Update to `rtl/prelude.myc` macro `cache`.
- Update to `rtl/series/mod.rs` to reflect new signature.
- Update to `ast/types.rs` to allow `series` to be indexed with an
  integer to retrieve its element type.

examples/sma.myc

@@ -11,11 +11,11 @@
   (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))
+  (macro cache [lookback src] `(do (def data (series ~lookback)) (pipe [~src] (fn [s] (do (push data s)))) data))
 
-;  (def bars (series 20 :float))
+;  (def bars (series 20))
 ;  (pipe [src] (fn [s] (do (push bars (.close s)))))
 
-  (cache 20 :float src)
+  (cache 20 src)
 
 )