Enhancement: Improve pruning performance and make pruning more customizable

The `prune` command is now much faster. This is especially the case for remote
repositories or repositories with not much data to remove.
Also the memory usage of the `prune` command is now reduced.
Restic used to rebuild the index from scratch after pruning. This is now
changed and the index rebuilding uses the information already known by `prune`.

By default, the `prune` command no longer removes all unused data. This
behavior can be fine-tuned by new options, like the acceptable amount of unused space or
the maximum size of data to reorganize. For more details, see
https://restic.readthedocs.io/en/stable/060_forget.html

Moreover, `prune` now accepts the `--dry-run` option and `forget --dry-run --prune`
also shows what `prune` would do.

Fixes several open issues, e.g.:
https://github.com/restic/restic/issues/1140
https://github.com/restic/restic/issues/1599
https://github.com/restic/restic/issues/1985
https://github.com/restic/restic/issues/2112
https://github.com/restic/restic/issues/2227
https://github.com/restic/restic/issues/2305

https://github.com/restic/restic/pull/2718
https://github.com/restic/restic/pull/2842