Enhancement: add compression statistics to the restic stats command

When executed in `raw-data` mode on a repository that supports compression, the `restic 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 and the total space saving, taking into account both
the compressed and uncompressed data if the repository is only partially compressed.

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