Add test for range propagation over shift operators

[emwap]

There is no test case for range propagation over shift operators. Also, the implementation should be updated to propagate over signed operands.

This blocks #13.

[pjonsson]

There are properties for shiftL/shiftR already defined in Test.hs (prop_shiftLU/prop_shiftRU) but they're not wired into the test harness. I didn't manage to wire them in when I tried though.

[emwap]

I have added these tests on the issue14 branch. However, the range propagation for shift operations is still lacking.

[pjonsson]

Doesn't the code I initially attached enable that range propagation? The tests you added pass in my tree.

[emwap]

Somewhat. rangeShiftLU improves the propagation if the first operand is unsigned, but it returns universal for signed operands.

The test case I just enabled only checks that the range is safe; i.e. that the result of evaluating the function stays within the propagated range.
It does not check that the range is tight which means that if the propagated range is universal the test will always pass.