Previously, mpz was restricted to using at most 15 bits in each digit,
where a digit was a uint16_t.

With this patch, mpz can use all 16 bits in the uint16_t (improvement
to mpn_div was required).  This gives small inprovements in speed and
RAM usage.  It also yields savings in ROM code size because all of the
digit masking operations become no-ops.

Also, mpz can now use a uint32_t as the digit type, and hence use 32
bits per digit.  This will give decent improvements in mpz speed on
64-bit machines.

Test for big integer division added.