add commented out debug statements

they are quite interesting to "follow" the algorithm as it searches for
a solution.

rust/solve/src/dpll.rs

    
    loop {
        if clauses.iter().all(|c| is_clause_satisfied(&vars, c.clone())) { // all clauses satisfied, success
            //println!("satisfied");
            return Some(vars.clone())
        } else if clauses.iter().any(|c| is_clause_conflict(&vars, c.clone())) { // a conflict exists, backtrack
            match stack.pop() {
                None => return None, // nothing to backtrack, no solution found
                Some((v, b)) => {
                    //println!("backtrack! {}", v);
                    vars.clone_from(&b);
                    vars.insert(v);
                }
            let cs = clauses.clone();
            let clause = cs.iter().find(|&c| is_clause_unit(&vars, c.clone())).unwrap();
            let unknown = *clause.iter().find(|&v| is_unknown(&vars, *v)).unwrap();
            //println!("propagate {} from {:?}", unknown, clause);
            vars.insert(unknown);
        } else { // none of the above, decide (guess) an unknown variable
            let mut unknown: Var = 0;
            }
            assert!(unknown != 0);
            stack.push((unknown, vars.clone()));
            //println!("decide {}", -unknown);
            vars.insert(-unknown);
        }
    }