The `IndexElementFn` trait and its associated `series_element_type`
function have been removed. The `TypeChecker` now directly inspects the
`StaticType::Series` variant to extract the element type when
propagating index constraints. This simplifies the type-checking logic
by removing an unnecessary layer of indirection and adhering to the
project rule of keeping the AST simple and pushing complexity into the
system.
Brummel
916460ab03