Deprecation

I've looked into using __attribute__ ((deprecated)).

See branch attribute-deprecated. I define _NETTLE_ATTRIBUTE_DEPRECATED in nettle-types.h (and _NETTLE_ATTRIBUTE_PURE, needed in a few places). For a start, I've marked the old aes and gcm-aes interfaces as deprecated, as discussed earlier, and the obsolete _rsa_blind and _rsa_unblind functions.

Some more functions are candidates for deprecation:

1. Everything in des-compat.h and md5-compat.h. Unless we think it can be deleted without deprecation; I'm not aware of any usage, and they made more sense back in 2001 when Nettle-1.0 was brand new.

2. The "general" hmac functions: hmac_set_key, hmac_update, hmac_digest.

Regards, /Niels