executor: move into tokio crate (#1702)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The executor implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

tokio-test/src/task.rs

@@ -1,6 +1,6 @@
 //! Futures task based helpers
 
-use tokio_executor::enter;
+use tokio::executor::enter;
 
 use pin_convert::AsPinMut;
 use std::future::Future;