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/process_issue_42.rs

@@ -0,0 +1,56 @@
+#![cfg(feature = "process")]
+#![cfg(unix)]
+#![warn(rust_2018_idioms)]
+
+use tokio::process::Command;
+use tokio::runtime::current_thread;
+
+use futures_util::future::FutureExt;
+use futures_util::stream::FuturesOrdered;
+use std::process::Stdio;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+use std::thread;
+use std::time::Duration;
+
+fn run_test() {
+    let finished = Arc::new(AtomicBool::new(false));
+    let finished_clone = finished.clone();
+
+    thread::spawn(move || {
+        let mut rt = current_thread::Runtime::new().expect("failed to get runtime");
+        let mut futures = FuturesOrdered::new();
+        rt.block_on(async {
+            for i in 0..2 {
+                futures.push(
+                    Command::new("echo")
+                        .arg(format!("I am spawned process #{}", i))
+                        .stdin(Stdio::null())
+                        .stdout(Stdio::null())
+                        .stderr(Stdio::null())
+                        .spawn()
+                        .unwrap()
+                        .boxed(),
+                )
+            }
+        });
+
+        drop(rt);
+        finished_clone.store(true, Ordering::SeqCst);
+    });
+
+    thread::sleep(Duration::from_millis(1000));
+    assert!(
+        finished.load(Ordering::SeqCst),
+        "FINISHED flag not set, maybe we deadlocked?"
+    );
+}
+
+#[test]
+fn issue_42() {
+    let max = 10;
+    for i in 0..max {
+        println!("running {}/{}", i, max);
+        run_test()
+    }
+}