Address comments from PR #64

Addresses @Adjective-Object 's comments from #64

Adds comments to UnusedFinder::refresh_* methods.
Removes redundant property UnusedFinder::flattened_walk_file_data to avoid information to not be synced in UnusedFinder::graph
Improves validation for max_iteration in bfs_step/frontier execution.

Adjective-Object / good-fences-rs-core