net: move into tokio crate (#1683)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The `net` implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

tokio/tests/uds_split.rs

@@ -0,0 +1,42 @@
+#![cfg(unix)]
+#![deny(warnings, rust_2018_idioms)]
+
+use tokio::net::UnixStream;
+use tokio::prelude::*;
+
+/// Checks that `UnixStream` can be split into a read half and a write half using
+/// `UnixStream::split` and `UnixStream::split_mut`.
+///
+/// Verifies that the implementation of `AsyncWrite::poll_shutdown` shutdowns the stream for
+/// writing by reading to the end of stream on the other side of the connection.
+#[tokio::test]
+async fn split() -> std::io::Result<()> {
+    let (mut a, mut b) = UnixStream::pair()?;
+
+    let (mut a_read, mut a_write) = a.split();
+    let (mut b_read, mut b_write) = b.split();
+
+    let (a_response, b_response) = futures::future::try_join(
+        send_recv_all(&mut a_read, &mut a_write, b"A"),
+        send_recv_all(&mut b_read, &mut b_write, b"B"),
+    )
+    .await?;
+
+    assert_eq!(a_response, b"B");
+    assert_eq!(b_response, b"A");
+
+    Ok(())
+}
+
+async fn send_recv_all(
+    read: &mut (dyn AsyncRead + Unpin),
+    write: &mut (dyn AsyncWrite + Unpin),
+    input: &[u8],
+) -> std::io::Result<Vec<u8>> {
+    write.write_all(input).await?;
+    write.shutdown().await?;
+
+    let mut output = Vec::new();
+    read.read_to_end(&mut output).await?;
+    Ok(output)
+}