New file.

(gcm_aes_set_key): New function.
(gcm_aes_set_iv): Likewise.
(gcm_aes_auth): Likewise.
(gcm_aes_encrypt): Likewise.
(gcm_aes_decrypt): Likewise.
(gcm_aes_digest): Likewise.

Rev: nettle/gcm-aes.c:1.1