[$] Modernizing swapping: the end of the swap map

The first installment in this seriesintroduced several data structures in the kernel's swap subsystem anddescribed work to replace some of those with a new "swap table" structure.The work did not stop there, though; there is more modernization of theswap subsystem queued for an upcoming development cycle, and even more formultiple kernel releases after that. Once that work is done, the swapsubsystem will be both simpler and faster than it is now.