Provide an ad-hoc debugging facility in serpent.c.
If the "#if 0" is changed to "#if 1", the key material, plaintext and
ciphertext of all Serpent operations is printed in hex to stderr.
We provide a new header file hexdebug.h to facilitate this. And we
use this new header file in the "#if 0" debugging in transform_setkey.
No functional change.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>