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, 1 insertions, 2 deletions

diff --git a/tokio/src/sync/oneshot.rs b/tokio/src/sync/oneshot.rs
index 3c757e9e..7b84f319 100644
--- a/tokio/src/sync/oneshot.rs
+++ b/tokio/src/sync/oneshot.rs
@@ -4,7 +4,6 @@ use crate::loom::cell::CausalCell;
 use crate::loom::sync::atomic::AtomicUsize;
 use crate::loom::sync::Arc;
 
-use futures_core::ready;
 use std::fmt;
 use std::future::Future;
 use std::mem::MaybeUninit;
@@ -225,7 +224,7 @@ impl<T> Sender<T> {
     /// }
     /// ```
     pub async fn closed(&mut self) {
-        use futures_util::future::poll_fn;
+        use crate::future::poll_fn;
 
         poll_fn(|cx| self.poll_closed(cx)).await
     }