When creating constant mpz's, the length of the mpz must be exactly how
many digits are used (not allocated) otherwise these numbers are not
compatible with dynamically allocated numbers.
Addresses issue #1448.
Previous to this patch, a big-int, float or imag constant was interned
(made into a qstr) and then parsed at runtime to create an object each
time it was needed. This is wasteful in RAM and not efficient. Now,
these constants are parsed straight away in the parser and turned into
objects. This allows constants with large numbers of digits (so
addresses issue #1103) and takes us a step closer to #722.
