support multiple spaces between variables

rust/solve/src/cnf.rs

	let mut clauses: Vec<Vec<i32>> = Vec::with_capacity(num_clauses as usize);
	for clause_line in clause_lines {
		let mut vars: Vec<i32> = clause_line.split(" ").map(|x| x.parse::<i32>().unwrap()).collect();
		let mut vars: Vec<i32> = clause_line.split(" ").filter(|x| x != &"").map(|x| x.parse::<i32>().unwrap()).collect();
		if vars.is_empty() {
			return Err("empty clause".to_string())
		}