Limit `futures` dependency to `Stream` via feature flag (#1774)

In an effort to reach API stability, the `tokio` crate is shedding its
_public_ dependencies on crates that are either a) do not provide a
stable (1.0+) release with longevity guarantees or b) match the `tokio`
release cadence. Of course, implementing `std` traits fits the
requirements.

The on exception, for now, is the `Stream` trait found in `futures_core`.
It is expected that this trait will not change much and be moved into `std.
Since Tokio is not yet going reaching 1.0, I feel that it is acceptable to maintain
a dependency on this trait given how foundational it is.

Since the `Stream` implementation is optional, types that are logically
streams provide `async fn next_*` functions to obtain the next value.
Avoiding the `next()` name prevents fn conflicts with `StreamExt::next()`.

Additionally, some misc cleanup is also done:

- `tokio::io::io` -> `tokio::io::util`.
- `delay` -> `delay_until`.
- `Timeout::new` -> `timeout(...)`.
- `signal::ctrl_c()` returns a future instead of a stream.
- `{tcp,unix}::Incoming` is removed (due to lack of `Stream` trait).
- `time::Throttle` is removed (due to lack of `Stream` trait).
-  Fix: `mpsc::UnboundedSender::send(&self)` (no more conflict with `Sink` fns).

1 files changed, 4 insertions, 3 deletions

diff --git a/tokio/tests/signal_ctrl_c.rs b/tokio/tests/signal_ctrl_c.rs
index ea4efaa2..13eeaa81 100644
--- a/tokio/tests/signal_ctrl_c.rs
+++ b/tokio/tests/signal_ctrl_c.rs
@@ -6,13 +6,13 @@ mod support {
 }
 use support::signal::send_signal;
 
-use tokio::prelude::*;
 use tokio::signal;
 use tokio::sync::oneshot;
+use tokio_test::assert_ok;
 
 #[tokio::test]
 async fn ctrl_c() {
-    let ctrl_c = signal::ctrl_c().expect("failed to init ctrl_c");
+    let ctrl_c = signal::ctrl_c();
 
     let (fire, wait) = oneshot::channel();
 
@@ -24,5 +24,6 @@ async fn ctrl_c() {
     });
 
     let _ = fire.send(());
-    let _ = ctrl_c.into_future().await;
+
+    assert_ok!(ctrl_c.await);
 }