Enhancement: Add compression statistics to the `stats` command

When executed with `--mode raw-data` on a repository that supports compression,
the `stats` command now calculates and displays, for the selected repository or
snapshots: the uncompressed size of the data; the compression progress
(percentage of data that has been compressed); the compression ratio of the
compressed data; the total space saving.

It also takes into account both the compressed and uncompressed data if the
repository is only partially compressed.

https://github.com/restic/restic/pull/3915