Feature Request: Use reflinks to copy profile data

When upgrading a snap, say, from version 123 to version 124, the contents of `~/snap/snapname/123` are copied to `~/snap/snapname/124`. (As far as I know, this is to allow rollbacks.)

However, the copy seems to be a "classic" one where every single bit is duplicated. On Cow filesystems such as btrfs, more efficient reflink copies are available (where data is only duplicated later when one of the copies is modified).

I propose snap use these. It'd make the update a lot quicker, and it'd save a lot of disk space.