DEV: h2: support hex-encoded data sequences in mkhdr

DEV: h2: support hex-encoded data sequences in mkhdr

For HPACK-encoded headers (particularly with huffman encoding), it's
really necessary to support hex sequences as they appear in RFC7541
examples, so let's support hex digit pairs with -R.

Now it's possible to do this to send GET https://www.example.com/ :

    (dev/h2/mkhdr.sh -t p; dev/h2/mkhdr.sh -t s;
     dev/h2/mkhdr.sh -t h -i 1 -f es,eh \
     -R '8286 8441 0f77 7777 2e65 7861 6d70 6c65 2e63 6f6d ') | nc 0 8080