Consider maximizing grid utilization

We currently maximize block utilization (taking the max threads), which may leave SMs underutilized. We should consider first selecting an optimal amount of blocks, before maximizing the thread could:

    config = launch_configuration(kernel.fun)
    threads = min(length(ps), config.threads)
    # XXX: this kernel performs much better with all blocks active
    blocks = max(cld(length(ps), threads), config.blocks)
    threads = cld(length(ps), blocks)

I'm sure this will lead to some kernels performing worse, though, but it's probably a good thing to test.