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/src/lib.rs

@@ -0,0 +1,8 @@
+#[cfg(feature = "tokio-executor")]
+pub use tokio_executor;
+
+#[cfg(feature = "tokio-net")]
+pub use tokio_net;
+
+#[cfg(feature = "tokio")]
+pub use tokio;