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).

4 files changed, 4 insertions, 5 deletions

diff --git a/tokio/src/sync/tests/loom_atomic_waker.rs b/tokio/src/sync/tests/loom_atomic_waker.rs
index 81e200ff..c148bcbe 100644
--- a/tokio/src/sync/tests/loom_atomic_waker.rs
+++ b/tokio/src/sync/tests/loom_atomic_waker.rs
@@ -1,6 +1,6 @@
 use crate::sync::task::AtomicWaker;
 
-use futures_util::future::poll_fn;
+use futures::future::poll_fn;
 use loom::future::block_on;
 use loom::sync::atomic::AtomicUsize;
 use loom::thread;
diff --git a/tokio/src/sync/tests/loom_mpsc.rs b/tokio/src/sync/tests/loom_mpsc.rs
index 748ae9e1..8fd6d14b 100644
--- a/tokio/src/sync/tests/loom_mpsc.rs
+++ b/tokio/src/sync/tests/loom_mpsc.rs
@@ -1,6 +1,6 @@
 use crate::sync::mpsc;
 
-use futures_util::future::poll_fn;
+use futures::future::poll_fn;
 use loom::future::block_on;
 use loom::thread;
 
diff --git a/tokio/src/sync/tests/loom_oneshot.rs b/tokio/src/sync/tests/loom_oneshot.rs
index 52104736..dfa7459d 100644
--- a/tokio/src/sync/tests/loom_oneshot.rs
+++ b/tokio/src/sync/tests/loom_oneshot.rs
@@ -1,6 +1,6 @@
 use crate::sync::oneshot;
 
-use futures_util::future::poll_fn;
+use futures::future::poll_fn;
 use loom::future::block_on;
 use loom::thread;
 use std::task::Poll::{Pending, Ready};
diff --git a/tokio/src/sync/tests/loom_semaphore.rs b/tokio/src/sync/tests/loom_semaphore.rs
index d14c7668..7b8de0f0 100644
--- a/tokio/src/sync/tests/loom_semaphore.rs
+++ b/tokio/src/sync/tests/loom_semaphore.rs
@@ -1,7 +1,6 @@
 use crate::sync::semaphore::*;
 
-use futures_core::ready;
-use futures_util::future::poll_fn;
+use futures::future::poll_fn;
 use loom::future::block_on;
 use loom::thread;
 use std::future::Future;