RFE: swap file support

[DemiMarie]

This is already on the TODO list, and feel free to close the issue if it won’t be implemented. That said, being able to support swap files that are encrypted, authenticated, and always-CoW would be nice. Presumably, the hard part would providing a write path that had no blocking memory allocations at all, and which could guarantee success even if all of the non-blocking allocations failed. dm-thin manages to support swap and CoW at the same time, so this should at least theoretically be possible.