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, 2 insertions, 3 deletions

diff --git a/tokio/tests/signal_usr1.rs b/tokio/tests/signal_usr1.rs
index 9b6a0dec..95fc6c10 100644
--- a/tokio/tests/signal_usr1.rs
+++ b/tokio/tests/signal_usr1.rs
@@ -6,18 +6,17 @@ mod support {
 }
 use support::signal::send_signal;
 
-use tokio::prelude::*;
 use tokio::signal::unix::{signal, SignalKind};
 use tokio_test::assert_ok;
 
 #[tokio::test]
 async fn signal_usr1() {
-    let signal = assert_ok!(
+    let mut signal = assert_ok!(
         signal(SignalKind::user_defined1()),
         "failed to create signal"
     );
 
     send_signal(libc::SIGUSR1);
 
-    let _ = signal.into_future().await;
+    signal.recv().await;
 }