add benchmark

src/benches/bench.rs

@@ -0,0 +1,20 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use maxwell::Universe;
+
+fn simple_system(w: u32, h: u32) -> Universe {
+    let mut universe = Universe::new(w, h);
+    universe.init(0.0, 0.0);
+    for _ in 0..100 {
+        universe.advance(0.01);
+    }
+    universe
+}
+
+fn performance_benchmark(c: &mut Criterion) {
+    c.bench_function("complete system", |b| {
+        b.iter(|| simple_system(black_box(25), black_box(30)))
+    });
+}
+
+criterion_group!(benches, performance_benchmark);
+criterion_main!(benches);