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).

examples/connect.rs

@@ -20,7 +20,7 @@ use tokio::io;
 use tokio::sync::{mpsc, oneshot};
 use tokio_util::codec::{FramedRead, FramedWrite};
 
-use futures::{SinkExt, Stream, StreamExt};
+use futures::{Stream, StreamExt};
 use std::env;
 use std::error::Error;
 use std::net::SocketAddr;
@@ -69,12 +69,14 @@ async fn run() -> Result<(), Box<dyn Error>> {
 
 // Temporary work around for stdin blocking the stream
 fn stdin() -> impl Stream<Item = Result<Vec<u8>, io::Error>> + Unpin {
-    let mut stdin = FramedRead::new(io::stdin(), codec::Bytes).map(Ok);
+    let mut stdin = FramedRead::new(io::stdin(), codec::Bytes);
 
-    let (mut tx, rx) = mpsc::unbounded_channel();
+    let (tx, rx) = mpsc::unbounded_channel();
 
     tokio::spawn(async move {
-        tx.send_all(&mut stdin).await.unwrap();
+        while let Some(res) = stdin.next().await {
+            let _ = tx.send(res);
+        }
     });
 
     rx