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, 1 insertions, 2 deletions

diff --git a/tokio/src/time/timeout.rs b/tokio/src/time/timeout.rs
index e1fdb252..2cc35082 100644
--- a/tokio/src/time/timeout.rs
+++ b/tokio/src/time/timeout.rs
@@ -5,14 +5,13 @@
 //! [`Timeout`]: struct.Timeout.html
 
 use crate::time::clock::now;
-use crate::time::Delay;
+use crate::time::{Delay, Duration, Instant};
 
 use futures_core::ready;
 use std::fmt;
 use std::future::Future;
 use std::pin::Pin;
 use std::task::{self, Poll};
-use std::time::{Duration, Instant};
 
 /// Allows a `Future` or `Stream` to execute for a limited amount of time.
 ///