[$] The proper time to split struct page

The page structure sits at the core of the kernel'smemory-management subsystem; one such structure exists for every page ofinstalled RAM. This structure is increasingly seen as a problem, though,and phasing it out is one of the many side projects associated with the folio conversion. One step in that directionis currently meeting some pushback from memory-management developers,though, who think that some of these changes are coming too soon.