add throughput benchmark

benches/throughput.rs

@@ -0,0 +1,14 @@
+#![feature(test)]
+
+extern crate test;
+extern crate want;
+
+#[bench]
+fn throughput(b: &mut test::Bencher) {
+    let (mut gv, mut tk) = want::new();
+
+    b.iter(move || {
+        tk.want();
+        assert!(gv.poll_want().unwrap().is_ready());
+    });
+}