I've added shadow versions of many operators that didn't have one. And, I've changed the behavior of various binary operators with more or fewer than 2 arguments to make the shadow and real versions more consistent and to make the behavior more logical in either case. For details, see the additions to NEWS and the new file tests/native_tests/operators.hy, which simultaneously tests shadow and real operators.
Although there are a lot of changes, I've put them all in one commit because they're interdependent.
