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/tests/net_driver.rs

@@ -61,7 +61,7 @@ fn test_drop_on_notify() {
         }
     }));
 
-    let _enter = tokio_executor::enter().unwrap();
+    let _enter = tokio::executor::enter().unwrap();
 
     {
         let handle = reactor.handle();