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/soa_series.myc

@@ -1,7 +1,7 @@
 ;; Benchmark: 1.1us
 ;; Benchmark-Repeat: 1877
 (do
-  (def my_ticks (series 100 {:price :float :volume :int :msg :text}))
+  (def my_ticks (series 100))
 
   (push my_ticks {:price 10.5 :volume 100 :msg "A"})
   (push my_ticks {:price 11.2 :volume 200 :msg "B"})