This commit makes sure that the value zero is always encoded in an mpz_t as
neg=0 and len=0 (previously it was just len=0).
This invariant is needed for some of the bitwise operations that operate on
negative numbers, because they cannot handle -0. For example
(-((1<<100)-(1<<100)))|1 was being computed as -65535, instead of 1.
Fixes issue #8042.
Signed-off-by: Damien George <damien@micropython.org>
