Thoughts on additional spilling layers

Have had a few thoughts about additional spilling layers we could benefit from:

* Defragmentation (perform a `memcpy` into larger and larger buffers to clean up and consolidate small buffers)
* Shared memory (also has the benefit of being accessible from other workers on the same node)
* mmap files (in-between shared memory and a file on disk; can close when memory usage is high; also like memory when using solid state)

Would be straightforward to add these to Zict without any new dependencies