add "scan" subcommand

Re-add the "scan" subcommand which scans a folder hierarchy for .AVB/BGB files and generates statistics on how many are corrupted (and in what way) and can be used for copying unique files and deduplicating collections to a managed folder hierarchy.

This command is absolutely the nastiest one in my dev tree, but it's super important to my own archival work!

unpolished development version is available at https://gitlab.com/nanoNago/cici/-/tree/devel?ref_type=heads