executor: move current-thread into crate (#1447)

The `CurrentThread` executor is exposed using a feature flag.

Refs: #1264

tokio-timer/tests/hammer.rs

@@ -1,7 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![feature(async_await)]
 
-use tokio_current_thread::CurrentThread;
+use tokio_executor::current_thread::CurrentThread;
 use tokio_executor::park::{Park, Unpark, UnparkThread};
 use tokio_timer::{Delay, Timer};