* gcm.c (gcm_set_key): Replaced context argument by a struct

gcm_key *.
(gcm_hash): Replaced context argument by a struct gcm_key * and a
pointer to the hashing state block.
(gcm_auth): Added struct gcm_key * argument.
(gcm_encrypt): Likewise.
(gcm_decrypt): Likewise.
(gcm_digest): Likewise.

* gcm.h (struct gcm_key): Moved the key-dependent and
message-independent state to its own struct.
(struct gcm_ctx): ... and removed it here.
(GCM_CTX): New macro.
(GCM_SET_KEY): Likewise.
(GCM_AUTH): Likewise.
(GCM_ENCRYPT): Likewise.
(GCM_DECRYPT): Likewise.
(GCM_DIGEST): Likewise.
(struct gcm_aes_ctx): New struct.

Rev: nettle/ChangeLog:1.142
Rev: nettle/gcm.c:1.12
Rev: nettle/gcm.h:1.7