We currently leak a pool page, because we cannot unlink the free objects
belonging to the page from the pool's free list.

Fix by having a per-page free list (containing objects only from that page).
The pages are themselves placed on a doubly linked list.  When we identify
an empty page, we can now easily drop it since the local free list only
point within the page.

We kept pool::_free pointing at a freed object.  Naturally things did not
work well.

Fix by nulling the pointer for now, however this leaks the rest of the free
list.

Must be large enough to hold a free_object.

Pages allocated with alloc_page() have no metadata, so we must format the
header before returing it to the free range tree.

This implementation stores small objects in pools of similar-sized objects,
while large objects are allocated using a first-fit algorithm.  There is also
a specialized interface for allocating aligned pages.