* aes.h (aes_encrypt, aes_decrypt): Declare ctx argument as const.

Also updated implementation.
* blowfish.h (blowfish_encrypt, blowfish_decrypt): Likewise.
* cast128.h (cast128_encrypt, cast128_decrypt): Likewise.
* serpent.h (serpent_encrypt, serpent_decrypt): Likewise.
* twofish.h (twofish_encrypt, twofish_decrypt): Likewise.

Rev: nettle/ChangeLog:1.91
Rev: nettle/aes-decrypt.c:1.2
Rev: nettle/aes-encrypt.c:1.2
Rev: nettle/aes.h:1.2
Rev: nettle/blowfish.c:1.3
Rev: nettle/blowfish.h:1.3
Rev: nettle/cast128.c:1.2
Rev: nettle/cast128.h:1.2
Rev: nettle/serpent.c:1.2
Rev: nettle/serpent.h:1.2
Rev: nettle/twofish.c:1.2
Rev: nettle/twofish.h:1.2