* gcm.c (gcm_gf_shift): Added a separate result argument.

(gcm_gf_mul): Compile bitwise version only when GCM_TABLE_BITS ==
0. Simplified interface with just two arguments pointing to
complete blocks.
(gcm_gf_shift_4, gcm_gf_shift_8): Renamed table-based functions, from...
(gcm_gf_shift_chunk): ... old name.
(gcm_gf_mul): Renamed both table-based versions and made the
argument types compatible with the bitwise gcm_gf_mul.
(gcm_gf_mul_chunk): ... the old name.
(gcm_set_key): Initialize the table using adds and shifts only.
When GCM_TABLE_BITS > 0, this eliminates the only use of the
bitwise multiplication.
(gcm_hash): Simplified, now that we have the same interface for
gcm_gf_mul, regardless of table size.

Rev: nettle/ChangeLog:1.140
Rev: nettle/gcm.c:1.11