remove threading workaround

as dpll is now non-recursive, we don't blow the stack anymore.

rust/solve/src/main.rs

use std::io;
use std::io::Read;
use std::thread;
use solve::cnf;
use solve::dpll;
		    println!("{:?}", clause);
		}
                thread::Builder::new().name("solver".to_string()).stack_size(100 * 1024 * 1024).spawn(move || {
                    match dpll::dpll(cnf.clauses) {
                        Some(bindings) => println!("satisfiable: {:?}", bindings),
                        None => println!("not satisfiable")
                    }
                }).unwrap().join();
                match dpll::dpll(cnf.clauses) {
                    Some(bindings) => println!("satisfiable: {:?}", bindings),
                    None => println!("not satisfiable")
                }
	    }
	    Err(e) => { println!("Error: {}", e) }
	},