mpz_limbs interface

[Zimmermann Paul]

>   PS: when doing "make check" with mpfr development version and the gmp snapshot
>   from last night, I get one failure:
>   
>   Seed GMP_CHECK_RANDOMIZE=1391280408 (include this in bug reports)
>   repl-vsnprintf.c:379: GNU MP assertion failed: len < total_width
>   /bin/bash: line 5:  7656 Aborted                 (core dumped) MPFR_QUIET=1 ${dir}$tst
>   FAIL: tprintf
>   
> I trust that you will debug this and report it to us if it is GMP's fault.

most likely it is a bug in MPFR or the way MPFR uses GMP, but in case it
still pops up when GMP 5.2 comes out, be sure we will debug and report!

Paul