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/fail/executor_without_current_thread.rs

@@ -0,0 +1,3 @@
+use tests_build::tokio_executor::current_thread;
+
+fn main() {}