Speed up hexyl

[sharkdp]

this PR:

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
hexyl data	588.3 ± 3.7	585.2	596.1	36.01
hexyl --color=never data	431.6 ± 7.0	425.2	447.5	26.42
hexdump -C data	16.3 ± 0.6	15.7	20.2	1.00
xxd data	706.3 ± 4.6	701.5	716.4	43.24

before:

Command	Mean [s]	Min [s]	Max [s]	Relative
hexyl data	1.031 ± 0.007	1.023	1.047	62.40
hexyl --color=never data	0.839 ± 0.005	0.834	0.848	50.79

benchmark command:

dd if=/dev/zero    bs=10M count=1 >  data
dd if=/dev/urandom bs=1k  count=1 >> data

hyperfine --warmup 5 \   
    'hexyl data' \
    'hexyl --color=never data' \
    'hexdump -C data' \
    'xxd data' \
    --export-markdown results.md

this means that we now clearly beat xxd (in this scenario) and can close #66 😄