runtime: merge multi & single threaded runtimes (#1716)

Simplify Tokio's runtime construct by combining both Runtime variants
into a single type. The execution style can be controlled by a
configuration setting on `Builder`.

The implication of this change is that there is no longer any way to
spawn `!Send` futures. This, however, is a temporary limitation. A
different strategy will be employed for supporting `!Send` futures.

Included in this patch is a rework of `task::JoinHandle` to support
using this type from both the thread-pool and current-thread executors.

1 files changed, 9 insertions, 2 deletions

diff --git a/tokio/tests/signal_multi_rt.rs b/tokio/tests/signal_multi_rt.rs
index 9ecf70ed..6a16dd88 100644
--- a/tokio/tests/signal_multi_rt.rs
+++ b/tokio/tests/signal_multi_rt.rs
@@ -7,7 +7,7 @@ mod support {
 use support::signal::send_signal;
 
 use tokio::prelude::*;
-use tokio::runtime::current_thread::Runtime;
+use tokio::runtime::Runtime;
 use tokio::signal::unix::{signal, SignalKind};
 
 use std::sync::mpsc::channel;
@@ -24,7 +24,7 @@ fn multi_loop() {
             .map(|_| {
                 let sender = sender.clone();
                 thread::spawn(move || {
-                    let mut rt = Runtime::new().unwrap();
+                    let mut rt = rt();
                     let _ = rt.block_on(async {
                         let signal = signal(SignalKind::hangup()).unwrap();
                         sender.send(()).unwrap();
@@ -45,3 +45,10 @@ fn multi_loop() {
         }
     }
 }
+
+fn rt() -> Runtime {
+    tokio::runtime::Builder::new()
+        .current_thread()
+        .build()
+        .unwrap()
+}