Prevent unnesting during generator creation

[Broes De Cat]

To create generators with bounds, unnestterms is called. This is possibly something left over from earlier code, but the resulting bdds are very complex resulting in considerable (x20) slowdown on some benchmarks. Solution: drop unnestterms and make evaluate structure smart so it can handle terms in formulas, for example by substituting them in the original bdd.

[Bart Bogaerts]

see 26bfd15

[Bart Bogaerts]

Note: three valued terms will still have to be unnested (because we might have done propagation on them)