Refactor: Move inlining logic to Inliner struct

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`.

src/ast/compiler/optimizer/mod.rs

@@ -1,7 +1,10 @@
 pub mod engine;
 pub mod folder;
+pub mod inliner;
 pub mod substitution_map;
+pub mod utils;
 
 pub use engine::Optimizer;
 pub use folder::Folder;
+pub use inliner::Inliner;
 pub use substitution_map::{SubstitutionMap, UsageInfo};