Introduces the `Inliner` struct to encapsulate logic related to value
and function inlining. This improves code organization and readability
within the optimizer engine.
Key changes include:
- Moving `is_inlinable_value` to the `Inliner` struct.
- Extracting beta-reduction preparation logic into
`Inliner::prepare_beta_reduction`.
- Moving parameter slot collection to
`Inliner::collect_parameter_slots`.
- Introducing `flatten_tuple` to a new `utils` module, used by both
`Folder` and `Inliner`.
Michael Schimmel