Extract common AST manipulation and folding logic into a new Folder
struct. This improves code organization and reusability. The Optimizer
now delegates these tasks to the Folder.
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.