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

[covert-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.