util/slab: re-design to allow migration between pools (v3)

This is basically a re-write of the slab allocator into a design where
multiple child pools are linked to a parent pool. The intention is that
every (GL, pipe) context has its own child pool, while the corresponding
parent pool is held by the winsys or screen, or possibly the GL share group.

The fast path is still used when objects are freed by the same child pool
that allocated them. However, it is now also possible to free an object in a
different pool, as long as they belong to the same parent. Objects also
survive the destruction of the (child) pool from which they were allocated.

The slow path will return freed objects to the child pool from which they
were originally allocated. If that child pool was destroyed, the corresponding
page is considered an orphan and will be freed once all objects in it have
been freed.

This allocation pattern is required for pipe_transfers that correspond to
(GL) buffer object mappings when the mapping is created in one context
which is later destroyed while other contexts of the same share group live
on -- see the bug report referenced below.

Note that individual drivers do need to migrate to the new interface in
order to benefit and fix the bug.

v2: use singly-linked lists everywhere
v3: use p_atomic_set for page->u.num_remaining

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97894

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