add multithreaded example

examples/multithreaded.rs

@@ -0,0 +1,8 @@
+
+use powlib::{gen::{POWRange, POWChallenge}, solver::POWSolver};
+
+fn main() {
+    let challenge = POWChallenge::make(POWRange::new(0, 20480));
+    let mut solver = POWSolver::new(challenge);
+    println!("Found {} with 8 threads", solver.solve_blocking(8, Some(|x| println!("{x}"))));
+}