Introduced 4-bit tables. Gives gmac performance of 45 cycles per

byte (still on intel x86_64).
* gcm.c (gcm_gf_shift): Renamed. Tweaked little-endian masks.
(gcm_rightshift): ... old name.
(gcm_gf_mul): New argument for the output. Added length argument
for one of the inputs (implicitly padding with zeros).
(shift_table): New table (in 4-bit and 8-bit versions), generated
by gcmdata.
(gcm_gf_shift_chunk): New function shifting 4 bits at
a time.
(gcm_gf_mul_chunk): New function processing 4 bits at a time.
(gcm_set_key): Generation of 4-bit key table.
(gcm_hash): Use tables, when available.

Rev: nettle/gcm.c:1.5