Re-implement extension-data-function, more clearly

[slankdev]

Extension-data's implementation is crude. We have to re-implement that.

[slankdev]

implementation example

pgen_arp pack;
pack.ARP.hwsrc = "";
pack.DATA[0] = 1;
pack.DATA[0] = 2;

char str = "test";
pgen_arp pack;
pack.ARP.hwsrc = "";
memcpy(pack.DATA, str, sizeof(str));

[slankdev]

implementation example

pgen_arp pack;
pack.ARP.hwsrc = "";
pack.DATA(str, sizeof(str));

[slankdev]

I didn't use these example. Implemented below.

implemented result

pgen_arp pack;
pack.ARP.hwsrc = "";
pack.DATA_set(str, sizeof(str));

pgen_hex(pack.DATA_get(), pack.DATA_len());