(nettle_mpz_sizeinbase_256_s): New function.

(nettle_mpz_sizeinbase_256_u): New name, was
nettle_mpz_sizeinbase_256.
(nettle_mpz_to_octets): New function.
(nettle_mpz_get_str_256): Handle negative numbers.
(nettle_mpz_from_octets): New function.
(nettle_mpz_set_str_256_u): New name, was nettle_mpz_set_str_256.
(nettle_mpz_init_set_str_256_u): New name, was
nettle_mpz_init_set_str_256.
(nettle_mpz_set_str_256_s): New function, handling negative two's
complement numbers.
(nettle_mpz_init_set_str_256_s): And an init variant.

Rev: src/nettle/bignum.c:1.5
Rev: src/nettle/bignum.h:1.6