Remove guarantees on node ordering from Bdd representation.

[daemontus]

In some cases, it is much more beneficial to produce Bdd nodes in a non-DFS order (for example #24, but other "custom" operations as well). Therefore in the future we should implement operations in such a way that they do not assume the Bdd to be sorted based on DFS post-order.

The other question is whether we can also lift the requirement of a Bdd being canonical, but that seems to be much harder to do, because some algorithms are easy exactly because Bdd is canonical. So for now, we only want to remove the ordering requirement.

In particular, this requires equivalence checks to be re-implemented, and to audit other algorithms to check whether they don't have a problem with different orderings.