introduce an alias for clauses

rust/solve/src/dpll.rs

type Var = i32;
type BoundVars = BTreeSet<Var>;
type Clause = Vec<Var>;
fn empty_vars() -> BoundVars {
    BTreeSet::new()
    vars
}
fn is_clause_satisfied(vars: BoundVars, clause: Vec<Var>) -> bool {
fn is_clause_satisfied(vars: BoundVars, clause: Clause) -> bool {
    for v in clause {
        if vars.contains(&v) {
            return true