objint_mpz: Quick&dirty implementation of bitwise operations.

Made solely to unbreak int-long.py test which in turn uncovered thinko
with implementation of inplace ops. On mpz level, bitwise ops implemented
only for same-sign numbers, and are not efficient (unconditional calling of
mpn_cmp() is apparently superfluous).
Merge pull request #364 from pfalcon/mpz-unbreak-int-long

objint_mpz: Quick&dirty implementation of bitwise operations.