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-build/tests/features.rs

@@ -0,0 +1,62 @@
+#![allow(unused_imports)]
+
+#[test]
+#[cfg(feature = "tokio-net")]
+fn net_default() {
+    use tests_build::tokio_net::driver::{set_default, Handle, Reactor, Registration};
+    use tests_build::tokio_net::util::PollEvented;
+}
+
+#[test]
+#[cfg(feature = "net-with-tcp")]
+fn net_with_tcp() {
+    use tests_build::tokio_net::tcp;
+}
+
+#[test]
+#[cfg(feature = "net-with-udp")]
+fn net_with_udp() {
+    use tests_build::tokio_net::udp;
+}
+
+#[test]
+#[cfg(feature = "net-with-uds")]
+fn net_with_uds() {
+    use tests_build::tokio_net::uds;
+}
+
+#[test]
+#[cfg(feature = "net-with-process")]
+fn net_with_process() {
+    use tests_build::tokio_net::process;
+}
+
+#[test]
+#[cfg(feature = "tokio-with-net")]
+fn tokio_with_net() {
+    // net is present
+    use tests_build::tokio::net;
+}
+
+#[test]
+fn compile_fail() {
+    let t = trybuild::TestCases::new();
+
+    #[cfg(feature = "executor-without-current-thread")]
+    t.compile_fail("tests/fail/executor_without_current_thread.rs");
+
+    #[cfg(feature = "macros-invalid-input")]
+    t.compile_fail("tests/fail/macros_invalid_input.rs");
+
+    #[cfg(feature = "net-no-features")]
+    {
+        t.compile_fail("tests/fail/net_without_tcp_missing_tcp.rs");
+        t.compile_fail("tests/fail/net_without_udp_missing_udp.rs");
+        t.compile_fail("tests/fail/net_without_uds_missing_uds.rs");
+    }
+
+    #[cfg(feature = "tokio-no-features")]
+    t.compile_fail("tests/fail/tokio_without_net_missing_net.rs");
+
+    drop(t);
+}