This is a total rewrite after having experimented with a variety of
techniques including a dual-allocator strategy for small and large size
classes where the large class allocator had a coalescing mechanism.
This allocator design is much closer to `mimalloc` in spirit and
performs almost twice as fast as the original bitmap-based design.
Additionally, several bugs have been fixed. The most important one being
the lock-free synchronization method used for remote frees. This design
uses a single atomic pointer that is doubly-tagged to pass remote frees
either to the heap or to the slab, depending on the ownership status.
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