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.

tests-integration/src/bin/test-cat.rs

@@ -0,0 +1,20 @@
+//! A cat-like utility that can be used as a subprocess to test I/O
+//! stream communication.
+
+use std::io;
+use std::io::Write;
+
+fn main() {
+    let stdin = io::stdin();
+    let mut stdout = io::stdout();
+    let mut line = String::new();
+    loop {
+        line.clear();
+        stdin.read_line(&mut line).unwrap();
+        if line.is_empty() {
+            break;
+        }
+        stdout.write_all(line.as_bytes()).unwrap();
+    }
+    stdout.flush().unwrap();
+}