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, 3 insertions, 3 deletions

diff --git a/tokio/tests/support/mock_pool.rs b/tokio/tests/support/mock_pool.rs
index acdb8dbc..e1fdb426 100644
--- a/tokio/tests/support/mock_pool.rs
+++ b/tokio/tests/support/mock_pool.rs
@@ -32,13 +32,13 @@ where
 }
 
 impl<T> Future for Blocking<T> {
-    type Output = T;
+    type Output = Result<T, io::Error>;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         use std::task::Poll::*;
 
         match Pin::new(&mut self.rx).poll(cx) {
-            Ready(Ok(v)) => Ready(v),
+            Ready(Ok(v)) => Ready(Ok(v)),
             Ready(Err(e)) => panic!("error = {:?}", e),
             Pending => Pending,
         }
@@ -50,7 +50,7 @@ where
     F: FnOnce() -> io::Result<T> + Send + 'static,
     T: Send + 'static,
 {
-    run(f).await
+    run(f).await?
 }
 
 pub(crate) fn len() -> usize {