Problem with large values in GMP...

[Torbjörn Granlund]

I don't think it is correct to make these type changes based on
_LONG_LONG_LIMB. We may want to extend mp_size_t and mp_size_tto to 64
bits on LLP64 systems and perhaps mp_bitcnt_t to 64 bits on any
_LONG_LONG_LIMB.

But any such change needs careful testing and code review to make sure
things work around the new borders (e.g. 2^32 bits and 2^31 limbs).
Just 'make check' is not an indicator of success.

We plan to collect many changes which break compatibility slightly for
GMP 7; we already consider changes to at leat mp_bitcnt_t.