Refactor: Move optimizer related types to dedicated module

This commit reorganizes the optimizer code by creating a new `optimizer`
module. The `Optimizer` struct and its related logic remain in
`optimizer/optimizer.rs`, while `SubstitutionMap` and `UsageInfo` are
moved to `optimizer/substitution_map.rs`.

This change improves code organization and makes it easier to manage the
optimizer's components.

src/ast/compiler/optimizer/mod.rs

@@ -0,0 +1,5 @@
+pub mod optimizer;
+pub mod substitution_map;
+
+pub use optimizer::Optimizer;
+pub use substitution_map::{SubstitutionMap, UsageInfo};