Better implementation of archive / file index (attachments in encryption)

The current implementation is rather naive. Should be rewritten using `pathlib` where possible. Only the file being read should be kept open. Possibly it should be refactored so that files are opened by the application and not internally within `covert.archive`, similarly to how the decryption works, where the archive only supplies filenames and the application (covert.cli) does all file I/O.