History:
For `countdown` we need:
[X] decrement register (`dec a`)
[X] Zero flag
[X] Relative jump on non-zero (`bne D`)
[X] Backward labels
For `memcpy` we need:
[X] cd register
[X] load register direct 'ld cd, NN'
[X] inc/dec reg pair (`inc cd`)
[X] Relative jump on zero (`beq D`)
[X] 8-bit compare immediate (`cmp b, N`)
[X] Store register indirect (`ld (cd), a`)
[X] load register indirect (`ld a, (cd)`)
[X] Unconditional relative jump (`bra D`)
[X] Forward labels
For `memtest` we need:
[X] memory write
[X] 16-bit compare immediate (`cmp cd, NN`)
[X] indirect inc/dec (`dec (cd)`)