best way to get mantissa from mpf_t

[Roger While]

 > For GMP 4.3, I will rename _mp_d to _mp_size and _mp_size to _mp_d, so
 > any code that plays with GMP internals will be duly punished.
 > Seriously, I will rename some fields in 4.3, with the hope that I get
 > feedback from those who depend on internals, since such usages suggest
 > weaknesses of GMP's interface.  I'd like to address those weaknesses.

Needed eg. for implementing mpz_get_ull, mpz_get_sll, mpz_set_ull, mpz_set_sll
for "long long" integers.

Roger