fix potassco/clasp#74 (#359)
fix potassco/clasp#74 (#359)

Function `Clasp::Enumerator::commitClause(lits)` expects literals to be
unique and clauses not to be tautological. This commit simply uses
`std::sort`, `std::unique` and `std::adjacent_find` to remove unnecessary
literals and detect tautological clauses.
potassco/clingo,potassco/clingo,potassco/clingo,potassco/clingo