[$] An operations structure for swap devices

One of the ideas raised at the for the swap subsystem. Like many parts of the
kernel, the swap layer evolved over time, with pieces being added as
needed; the end result of this evolution is rarely what one would expect
had the subsystem been designed today. The interface between the swap
layer and the devices it uses is just one example. It appears that one
result of the swap subsystem's evolution — the lack of an abstraction layer
to interface with underlying storage — will soon be addressed, but in a
different way than was initially envisioned.