* aes.h (aes_invert_key): Declare it.

* aes-set-decrypt-key.c (aes_invert_key): New function, key
inversion code extracted from aes_set_decrypt_key.
(aes_set_decrypt_key): Use aes_invert_key.

Rev: nettle/ChangeLog:1.97
Rev: nettle/aes-set-decrypt-key.c:1.2
Rev: nettle/aes.h:1.3