Move stream items into tokio-stream (#3277)

This change removes all references to `Stream` from
within the `tokio` crate and moves them into a new
`tokio-stream` crate. Most types have had their
`impl Stream` removed as well in-favor of their
inherent methods.

Closes #2870

tokio-stream/src/empty.rs

@@ -0,0 +1,50 @@
+use crate::Stream;
+
+use core::marker::PhantomData;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Stream for the [`empty`](fn@empty) function.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct Empty<T>(PhantomData<T>);
+
+impl<T> Unpin for Empty<T> {}
+unsafe impl<T> Send for Empty<T> {}
+unsafe impl<T> Sync for Empty<T> {}
+
+/// Creates a stream that yields nothing.
+///
+/// The returned stream is immediately ready and returns `None`. Use
+/// [`stream::pending()`](super::pending()) to obtain a stream that is never
+/// ready.
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// use tokio_stream::{self as stream, StreamExt};
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut none = stream::empty::<i32>();
+///
+///     assert_eq!(None, none.next().await);
+/// }
+/// ```
+pub const fn empty<T>() -> Empty<T> {
+    Empty(PhantomData)
+}
+
+impl<T> Stream for Empty<T> {
+    type Item = T;
+
+    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
+        Poll::Ready(None)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
+}