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, 7 insertions, 8 deletions

diff --git a/tokio/src/io/mod.rs b/tokio/src/io/mod.rs
index 9e0567de..3f95c330 100644
--- a/tokio/src/io/mod.rs
+++ b/tokio/src/io/mod.rs
@@ -45,19 +45,18 @@ pub use self::async_read::AsyncRead;
 mod async_write;
 pub use self::async_write::AsyncWrite;
 
-#[allow(clippy::module_inception)] // TODO: remove
 #[cfg(feature = "io-util")]
-mod io;
+pub mod split;
 #[cfg(feature = "io-util")]
-pub use self::io::{
-    copy, empty, repeat, sink, AsyncBufReadExt, AsyncReadExt, AsyncWriteExt, BufReader, BufStream,
-    BufWriter, Copy, Empty, Repeat, Sink, Take,
-};
+pub use self::split::split;
 
 #[cfg(feature = "io-util")]
-pub mod split;
+mod util;
 #[cfg(feature = "io-util")]
-pub use self::split::split;
+pub use self::util::{
+    copy, empty, repeat, sink, AsyncBufReadExt, AsyncReadExt, AsyncWriteExt, BufReader, BufStream,
+    BufWriter, Copy, Empty, Lines, Repeat, Sink, Split, Take,
+};
 
 // TODO: These should not be guarded by `fs`