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.

2 files changed, 1 insertions, 28 deletions

diff --git a/tokio-test/tests/block_on.rs b/tokio-test/tests/block_on.rs
index c361d500..3c0fe32f 100644
--- a/tokio-test/tests/block_on.rs
+++ b/tokio-test/tests/block_on.rs
@@ -1,10 +1,8 @@
 #![warn(rust_2018_idioms)]
 
-use tokio::time::delay;
+use tokio::time::{delay, Duration, Instant};
 use tokio_test::block_on;
 
-use std::time::{Duration, Instant};
-
 #[test]
 fn async_block() {
     assert_eq!(4, block_on(async { 4 }));
diff --git a/tokio-test/tests/clock.rs b/tokio-test/tests/clock.rs
deleted file mode 100644
index d9d2fcfc..00000000
--- a/tokio-test/tests/clock.rs
+++ /dev/null
@@ -1,25 +0,0 @@
-#![warn(rust_2018_idioms)]
-
-use tokio::time::delay;
-use tokio_test::clock::MockClock;
-use tokio_test::task;
-use tokio_test::{assert_pending, assert_ready};
-
-use std::time::{Duration, Instant};
-
-#[test]
-fn clock() {
-    let mut mock = MockClock::new();
-
-    mock.enter(|handle| {
-        let deadline = Instant::now() + Duration::from_secs(1);
-        let mut delay = task::spawn(delay(deadline));
-
-        assert_pending!(delay.poll());
-
-        handle.advance(Duration::from_secs(2));
-
-        assert!(delay.is_woken());
-        assert_ready!(delay.poll());
-    });
-}