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).

tokio-util/Cargo.toml

@@ -32,7 +32,7 @@ pin-project = "0.4"
 tokio = { version = "=0.2.0-alpha.6", path = "../tokio" }
 tokio-test = { version = "=0.2.0-alpha.6", path = "../tokio-test" }
 
-futures-util = "0.3.0"
+futures = "0.3.0"
 
 [package.metadata.docs.rs]
 all-features = true

tokio-util/src/codec/length_delimited.rs

@@ -42,6 +42,7 @@
 //! use tokio::prelude::*;
 //! use tokio_util::codec::{Framed, LengthDelimitedCodec};
 //!
+//! use futures::SinkExt;
 //! use bytes::Bytes;
 //!
 //! async fn write_frame<T>(io: T) -> Result<(), Box<dyn std::error::Error>>

tokio-util/tests/framed.rs

@@ -5,6 +5,7 @@ use tokio_test::assert_ok;
 use tokio_util::codec::{Decoder, Encoder, Framed, FramedParts};
 
 use bytes::{Buf, BufMut, BytesMut, IntoBuf};
+use futures::StreamExt;
 use std::io::{self, Read};
 use std::pin::Pin;
 use std::task::{Context, Poll};

tokio-util/tests/framed_read.rs

@@ -1,11 +1,12 @@
 #![warn(rust_2018_idioms)]
 
-use tokio::prelude::*;
+use tokio::io::AsyncRead;
 use tokio_test::assert_ready;
 use tokio_test::task;
 use tokio_util::codec::{Decoder, FramedRead};
 
 use bytes::{Buf, BytesMut, IntoBuf};
+use futures::Stream;
 use std::collections::VecDeque;
 use std::io;
 use std::pin::Pin;

tokio-util/tests/length_delimited.rs

@@ -1,7 +1,6 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::io::{AsyncRead, AsyncWrite};
-use tokio::prelude::*;
 use tokio_test::task;
 use tokio_test::{
     assert_err, assert_ok, assert_pending, assert_ready, assert_ready_err, assert_ready_ok,
@@ -9,7 +8,7 @@ use tokio_test::{
 use tokio_util::codec::*;
 
 use bytes::{BufMut, Bytes, BytesMut};
-use futures_util::pin_mut;
+use futures::{pin_mut, Sink, Stream};
 use std::collections::VecDeque;
 use std::io;
 use std::pin::Pin;

tokio-util/tests/udp.rs

@@ -3,10 +3,10 @@ use tokio_util::codec::{Decoder, Encoder};
 use tokio_util::udp::UdpFramed;
 
 use bytes::{BufMut, BytesMut};
-use futures_util::future::try_join;
-use futures_util::future::FutureExt;
-use futures_util::sink::SinkExt;
-use futures_util::stream::StreamExt;
+use futures::future::try_join;
+use futures::future::FutureExt;
+use futures::sink::SinkExt;
+use futures::stream::StreamExt;
 use std::io;
 
 #[tokio::test]