Size of each block in an sparse bitset.
The block size can be any positive number, but for good performance you probably want to use the maximal number such that:
On CCL and SBCL, 60 is the right number. For other Lisps, 60 is logically fine, but other numbers may give you better performance.
Note: if you change this, you may wish to also write a corresponding, optimized version of sbitset-pair-members.