Wishlist: Report size of backup (collection-status,list-files,...)

When restoring, it would be nice to know the size of the backup. I know this is difficult because everything is gzipped up. But I imagine either there's a decent estimation that could be provided (the reported size doesn't need to be 100% accurate, but hopefully errs on the side of over-reporting), or duplicity could keep track via its metadata as incrementals are added.

Once duplicity knows this, it can give an error if the destination is not large enough to contain it.

Additionally, it would be nice if it was reported in, say, a collection-status so that wrappers like Deja Dup could meaningfully use it.