box: allow slab_alloc_granularity = 4 back

It was disabled in the commit ccfef5fd ("memtx: ignore
slab_alloc_granularity < machine word size") to postpone addressing
unaligned objects issues.

Add PACKED attribute to inform compiler of unaligned objects. We also
need to introduce `struct stailq_entry_ptr` to be able to inform compiler
that `last` pointer unaligned too.

Closes #7422

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