remove two more uses of `.clone`

know your type system?  and/or the borrow checker.  this makes the code
faster again, about 0.3 to 0.4 seconds on the hole6 example.  (from ~3.45
to ~3.05.)

rust/solve/src/dpll.rs

                }
            }
        } else if clauses.iter().any(|c| is_clause_unit(&vars, c)) { // a unit clause exists, propagate
            let cs = clauses.clone();
            let clause = cs.iter().find(|&c| is_clause_unit(&vars, c)).unwrap();
            let clause = clauses.iter().find(|&c| is_clause_unit(&vars, c)).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;
            for c in clauses.clone() {
            for c in &clauses {
                match c.iter().find(|&v| is_unknown(vars, *v)) {
                    Some(&u) => { unknown = u; break }
                    None => continue