Rework interface for RSA private key operations
Nettle's RSA interfaces has evolved over the years, and we now have, e.g., rsa_decrypt, rsa_decrypt_tr, rsa_sec_decrypt. That is unnecessarily hard to both understand and maintain.
In addition, it would be good to have interfaces with octet strings as input and output, rather than mpz_t, and exclusively use gmp's mpn_ functions internally.
For any practical transition to a new interface, it may be necessary to introduce rsa2_ functions and types.