reorganize modules (#1766)

This patch started as an effort to make `time::Timer` private. However, in an
effort to get the build compiling again, more and more changes were made. This
probably should have been broken up, but here we are. I will attempt to
summarize the changes here.

* Feature flags are reorganized to make clearer. `net-driver` becomes
  `io-driver`. `rt-current-thread` becomes `rt-core`.

* The `Runtime` can be created without any executor. This replaces `enter`. It
  also allows creating I/O / time drivers that are standalone.

* `tokio::timer` is renamed to `tokio::time`. This brings it in line with `std`.

* `tokio::timer::Timer` is renamed to `Driver` and made private.

* The `clock` module is removed. Instead, an `Instant` type is provided. This
  type defaults to calling `std::time::Instant`. A `test-util` feature flag can
  be used to enable hooking into time.

* The `blocking` module is moved to the top level and is cleaned up.

* The `task` module is moved to the top level.

* The thread-pool's in-place blocking implementation is cleaned up.

* `runtime::Spawner` is renamed to `runtime::Handle` and can be used to "enter"
  a runtime context.

1 files changed, 2 insertions, 2 deletions

diff --git a/tokio/tests/rt_common.rs b/tokio/tests/rt_common.rs
index d6c1b1fd..73982ced 100644
--- a/tokio/tests/rt_common.rs
+++ b/tokio/tests/rt_common.rs
@@ -264,14 +264,14 @@ rt_test! {
     #[test]
     fn spawn_from_other_thread() {
         let mut rt = rt();
-        let sp = rt.spawner();
+        let handle = rt.handle().clone();
 
         let (tx, rx) = oneshot::channel();
 
         thread::spawn(move || {
             thread::sleep(Duration::from_millis(50));
 
-            sp.spawn(async move {
+            handle.spawn(async move {
                 assert_ok!(tx.send(()));
             });
         });