Implement stage B0 of PEP 237: add warnings for operations that currently return inconsistent results for ints and longs; in particular: hex/oct/%u/%o/%x/%X of negative short ints, and x<<n that either loses bits or changes sign. (No warnings for repr() of a long, though that will also change to lose the trailing 'L' eventually.) This introduces some warnings in the test suite; I'll take care of those later.