Extended testsuite uncovers flatten problems

[IgnaceBleukx]

I extended the test_constraints.py testsuite (in branch "extended_testsuite") this uncovers a range of errors related to boolean vs integer comparisons.
Bugs pop up in all solvers using the flatten transformation, so very likely this should be catched there.

As discusses offline, fixing this might have the need to introduce the unary "Not" operator to wrap around expressions instead of using == 0 to negate boolean expressions in general.

[tias]

Yes to introducing 'Not' operator after all. And then we should take negate_normal out of flatten.py and make it a separate transformation 'negation' that pushes the negations down... will enable a few more optimisations in flatten, and eliminate the current ==0 !=1 weird checks too.

[Wout4]

we now have push_down_negation and the not-operator