ossl_to_hex
ossl_to_hex(hextmp, *p);
ossl_to_hex(buf, a->data[i]);
ossl_to_hex(buf, a->data[i]);
p += ossl_to_hex(p, v);
p += ossl_to_hex(p, buf[i]);
q += ossl_to_hex(q, buf[i]);
p += ossl_to_hex(p, n);
p += ossl_to_hex(p, n);
size_t ossl_to_hex(char *buf, uint8_t n);