support comments and empty lines

rust/solve/src/cnf.rs

/// line specifies this.  Each following line represents a clause with possibly
/// negated literals, terminated by 0 and a newline.
pub fn parse_dimac(dimac: &str) -> Result<CNF, String> {
	let mut lines = dimac.lines();
	let mut lines = dimac.lines().filter(|l| !(l.starts_with("c") || l.trim() == ""));
	let mut num_vars;
	let mut num_clauses;