Demand paging

See merge request !238

Since these are stored in arrays that are typically 512 KiB each
(i.e. which in turn typically store metadata for 128 MiB), they will
certainly be a page-size multiple, and large contiguous allocations are
allowed at least at boot time. Not only is the linked-list allocator
less efficient for that, but this change will also help reduce TLB
overhead.

This might not be the most ideal solution, since a GiB grant can be
EBUSY blocked if a single page is used by an indefinitely-blocking
UserScheme. But, the alternatives would impose lots of additional
complexity, such as increasing the PageInfo size, adding more
refcounting arrays to grants, etc.

This enables proper CoW ^ shared enforcement, so that e.g. acquiring a
new CoW page from already shared memory, or vice versa, will enforce
that they aren't both simultaneously.