make opslot_slab an offset in current slab

make opslot_slab an offset in current slab

Each OPSLOT allocated within an OPSLAB contains a pointer, opslot_slab,
which points back to the first (head) slab of the slab chain (i.e. not
necessarily to the slab which the op is contained in).

This commit changes the pointer to be a 16-bit offset from the start of
the current slab, and adds a pointer at the start of each slab which
points back to the head slab.

The mapping from an op to the head slab is now a two-step process: use
the op's slot's opslot_offset field to find the start of the current
slab, then use that slab's new opslab_head pointer to find the head
slab.

The advantage of this is that it reduces the storage per op.  (It
probably doesn't make any practical difference yet, due to alignment
issues, but that will will be sorted shortly in this branch.)