Add enumerate combinator to Stream (#832)

tokio-timer/tests/enumerate.rs

@@ -0,0 +1,31 @@
+extern crate futures;
+extern crate tokio;
+extern crate tokio_executor;
+extern crate tokio_timer;
+
+#[macro_use]
+mod support;
+
+use futures::prelude::*;
+use futures::sync::mpsc;
+use tokio::util::StreamExt;
+
+#[test]
+fn enumerate() {
+    use futures::*;
+
+    let (mut tx, rx) = mpsc::channel(1);
+
+    std::thread::spawn(|| {
+        for i in 0..5 {
+            tx = tx.send(i * 2).wait().unwrap();
+        }
+    });
+
+    let mut result = rx.enumerate().collect();
+    assert_eq!(
+        result.wait(),
+        Ok(vec![(0, 0), (1, 2), (2, 4), (3, 6), (4, 8)])
+    );
+
+}