Refactor to prepare for unrolling of inner loop
Cache-in-register (pseudo-cache) as bool template parameter
Privatized histograms - unrolled version
Transpose the final summation (have 64 threads each compute 4
of the block's histogram elements to add, instead of doing
64 reductions across the 64 privatized elements for each
histogram).  This is more work-efficient.
Rename PrivatizedPerThread32Unroll to PrivatizedPerThread4x32