Function sbox_loader_kernel 

pub fn sbox_loader_kernel(
    _clock_reset: ClockReset,
    _input: (),
    q: SboxLoaderQ,
) -> (LoaderOut, SboxLoaderD)

Read S(i) one cycle, write it the next.

Inputs: only q (the counter, the pending write and the ROM output). Outputs: the LoaderOut for this cycle and d (the next counter, the next pending write and the next ROM address). A two-stage pipeline: stage 1 reads the ROM, stage 2 presents the write.