Remove redundant line empty when we eliminate negations

[josevalim]

No description provided.

[gldubc]

Removing tuple_dnf_union in tuple_eliminate_single_negation is fine because tuple_elim_size(n, m, tag, elements, neg_tag) and tuple_elim_content([], tag, elements, neg_elements) generate
tuples of different sizes, so they are disjoint.

But once tuple_bdd_to_dnf_no_negations/1 no longer unions the generated lines, different BDD lines can produce structural duplicates. That is semantically fine, but it can regress singleton?/1, because each_singleton?(:tuple, bdd) expects the DNF to contain exactly one tuple line for a singleton. I put an example of that.

So I think we should keep the optimization inside tuple_eliminate_single_negation, but still deduplicate at the final no-negations DNF boundary (via Enum.uniq()). An alternative is to do it directly in each_singleton(:tuple, bdd).

[josevalim]

@gldubc we can do a simple dedup that deduplicate only the next immediate element!

[gldubc]

Sounds good!

[josevalim]

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