Brummel
93a85cd15c
Introduce `IndexElementFn` as a type alias for a function that maps a `StaticType` to an optional `StaticType` representing its element type. This allows the `TypeChecker` to determine the element type of indexable types like `Series` without needing to be aware of their specific implementations. The `series_element_type` function is created and registered with the `TypeChecker` to handle `Series` types. This design centralizes type-specific logic for indexable types, making the `TypeChecker` more generic and extensible for future indexable types such as `Stream`.